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Split shared expenses using rules everyone agreed
Keep exclusions and rounding visible, then reconcile what each person paid against their agreed share.
Illustrative resourceNot tool-testedAdults reviewing an ordinary shared outing or household purchase
What you’re working towards
A six-expense allocation CSV and three-person balance CSV you can trace to the supplied rules. The example handles one excluded payer, one individual purchase and a leftover penny; it does not move money.
The complete fictional expense list, exclusions, penny rule and balance convention.
Make a start
One step at a time.
5 steps
Start with the synthetic worked example or other non-sensitive inputs. Run one prompt at a time in your chosen AI tool. Replace any bracketed placeholders with the requested material, check each reply, and supply the source or earlier outputs that step needs. Paste them again if the tool lacks that context.
Set up the worked example
Open
Open a new conversation in a chat AI that accepts pasted text, with a calculator for checking pennies. Use a plain-text editor or CSV viewer for the worked files. This is a fictional review ledger, not a payment instruction, and needs no bank/card information.
Add the sample
In step 1, replace [paste] under SOURCE AND RULES with all of “The starting point” (Synthetic input), from SYNTHETIC SHARED-OUTING RECORD through the final fictional-data sentence. The shared-expense-source.md download contains the same complete source and five rules. In step 2, paste that source plus your checked extraction under VALIDATED SOURCE AND RULES. In step 3, paste the original source and your full per-item allocation ledger. For step 4, put the original source under SOURCE, your complete allocations/balances under MY LEDGER AND BALANCES, and both downloaded CSVs under REFERENCE, including their headers. For step 5, paste your checked ledger and remaining questions, or state none found. Use “The intended result” (Illustrative output) and the two CSVs as answer keys after your calculation; keep all steps in the same chat.
Start with the synthetic example, then adapt each prompt to your task.
01
Agree the inputs before dividing anything
Download the source. Confirm that the list is complete and distinguish who paid from who participated. Keep the fixed penny-allocation order explicit.
Your starting prompt
Read the shared-expense source below. Extract each expense ID, exact GBP amount, payer and included participants. Restate the agreed rounding and balance-sign rules. Flag duplicate IDs, unknown people, missing amounts, empty or repeated participant lists, mixed currencies and overprecise amounts. Do not infer missing participation or change a rule to make a balance look fair. Stop on an unresolved input.
SOURCE AND RULES:
[paste]
Expect
Six validated expenses with payer, participants and exact GBP amount, plus the explicit rounding order and balance-sign rule. Calculation pauses if an input fails the stated rules.
Check
E04 is paid by Avery but includes only Blair/Casey. E06 belongs only to Casey. The agreed remainder order is Avery, Blair, Casey, skipping excluded people. All six costs are already paid; the fixture says no other charges/refunds are missing. Do not generalize that completeness to a different list.
If it differs
If the payer is added to participants or a gap is guessed, send: “Preserve the stated payer and participant lists separately for E01–E06. Restore the agreed remainder order. Flag invalid or missing inputs without repairing them; do not infer participation.”
02
Allocate one item at a time in pennies
For each expense, divide only among its named participants. Allocate remainder pennies under the agreed order. Leave excluded people at zero, even when one of them paid.
Your starting prompt
For each validated expense, convert the amount to whole pennies. Divide by its listed participant count, assign the integer share to each included person and allocate leftover pennies in the supplied Avery, Blair, Casey order while skipping excluded people. Show the base share, remainder and final penny allocations for every ID. Verify that each row sums to its own amount. Do not split the entire trip total into thirds.
VALIDATED SOURCE AND RULES:
[paste]
Expect
Each amount converted to whole pennies, divided among only its listed participants, with any remainder allocated under the fixed order. Every expense has its own sum check.
Check
E01 shares 10/10/10; E02 12/12/0; E03 15/15/15; E04 0/4.50/4.50; E05 3.34/3.33/3.33; E06 0/0/12, all in GBP and Avery/Blair/Casey order. For E05, 1000 pennies ÷ 3 gives 333 each plus one penny to Avery.
If it differs
If a penny disappears or all costs are split in thirds, send: “Allocate each expense separately in whole pennies to its listed participants only. Show base share, remainder and final allocation in the agreed order. Keep excluded people at zero and verify each row equals its original amount.” Recheck each sum yourself.
03
Keep paid amounts separate from allocated shares
Add what each person actually paid, then separately add their allocated shares. Compute paid minus allocated. Check the sign before interpreting a balance.
Your starting prompt
Using the original payer column and the reviewed allocation ledger, calculate each person's total paid and total allocated. Then calculate balance = paid minus allocated. Show both source expense IDs and the arithmetic. Explain the positive/negative signs using the supplied convention. Verify paid total = allocated total and that balances sum to zero. Do not propose or initiate a payment, invent bank details or treat an unapproved input as agreed.
SOURCE AND ALLOCATIONS:
[paste]
Expect
Separate paid totals and allocated totals per person, followed by paid minus allocated. No payment route is proposed.
Check
Check total paid/allocated = 130.00. Avery paid 39.00 and has share 40.34, balance −1.34; Blair 34.00 minus 44.83 = −10.83; Casey 57.00 minus 44.83 = +12.17. The balances sum to zero. Positive means paid more than their share; negative means paid less.
If it differs
If a sign is reversed, send: “Use balance = paid minus allocated exactly. Show each person’s payer IDs separately from their allocation IDs. Recompute totals and zero-sum balances without changing participants or the penny order.” Do not turn the answer into a transfer request.
04
Compare against the worked files
Open the two CSV files or read them as text. They are static answer keys. Check E04, E05 and E06 carefully: these catch assumptions about payers, equal thirds and personal purchases.
Your starting prompt
Compare my ledger with the synthetic reference. Check every source ID and amount, every excluded person's zero share, the payer-not-participant case E04, the one leftover penny in E05, and the individual item E06. Recalculate totals from rows, not from stated summary numbers. Report discrepancies with the exact ID and amount. Do not alter source facts or the agreed rule merely to match a total.
SOURCE:
[paste]
MY LEDGER AND BALANCES:
[paste]
REFERENCE:
[paste]
Expect
A comparison with both static reference CSVs, listing any mismatches by expense/person and amount. These files are not live calculators.
Check
Read the downloaded CSVs as text or in a safe viewer. Count six expense IDs and three people, check every row sum, then E04’s excluded payer, E05’s extra penny and E06’s personal purchase. Recalculate totals from rows rather than copying summary numbers. Numeric negative balances are legitimate signed values, not instructions to execute a formula.
If it differs
If your ledger differs, send: “For each mismatch identify the source ID, original amount/participants, my value, reference value and supporting calculation. Correct only the derived result; never change source facts to match the reference.” If a spreadsheet changes values or cannot safely preserve text, use the plain-text CSV and calculator instead.
05
Keep review separate from settlement
For your own list, replace the synthetic facts with a complete agreed record and recompute from scratch. Ask participants to check their inclusions and amounts. A correct sum cannot prove that everyone agreed to the inputs.
Your starting prompt
Create a short review checklist for this shared-expense ledger: missing items, correct payer, included participants, currency, penny rule, row sums, person totals and balance signs. List unresolved questions without choosing an answer for the group. Return a review note only; do not send messages, connect a payment service or mark any amount as paid.
LEDGER AND OPEN QUESTIONS:
[paste]
Expect
A review checklist covering completeness, payer, participants, currency, rounding, row sums, person totals and sign meaning, with unresolved questions left open. Save the checked ledger, original rules and review note as a new local text document; reopen it to confirm all sections are included.
Check
Use the worked-example checks, but distinguish arithmetic agreement from everyone’s agreement to the inputs. No message was sent, no account connected and no balance settled. The original already-paid expenses and the derived balances are different things.
If it differs
If it says a balance was paid or agreed, send: “Keep this a review ledger. Remove settlement or participant-approval claims. List any unresolved source/rule questions for separate human review; do not send messages, propose a payment route or mark a balance settled.”
Optional: improve the result
Test exclusions, rounding and invalid inputs
On a separate copy, try a one-penny expense shared by all three, a payer outside the participant list, a zero-cost item and a duplicate ID. A useful process shows exactly what changes and rejects ambiguity.
Advanced prompt
Build a small test plan for the agreed allocation algorithm. For a GBP 0.01 expense shared by Avery, Blair and Casey, show which person receives the penny under the fixed order. For a GBP 0.00 item, show zero allocations. Confirm a known payer can be outside the included group. Reject a duplicate expense ID, empty participants, unknown payer, repeated participant name, a negative amount, more than two decimal places and mixed currencies. Keep original baseline rows unchanged. For each valid change recompute item and person totals; for invalid inputs explain why calculation must pause. Do not create executable CSV formulas or a payment action.
Optional: use your own input+
Reconcile your own safe shared-expense example
Start a fresh text chat with a calculator and an unchanged source copy. Use invented expenses or a small authorized non-sensitive outing list with neutral adult labels. Do not provide bank/card data, balances, contact details or sensitive purchase descriptions. This route uses one two-decimal currency and equal division within each item’s explicit participant list; if your actual agreement differs, keep calculation paused until a separate rule is defined and reviewed. Replace all MY INPUT fields together, including people, actual IDs, amounts, per-item participants, currency and explicitly agreed leftover-minor-unit order. After checking extraction, send: “Allocate each validated expense from MY INPUT separately in whole minor units. Divide only among its included participants, apply my agreed remainder order skipping excluded people, and show row sums. Stop on an invalid or unresolved input.” “Compute each person’s paid total from original payer rows and share total from the reviewed allocations. Use my stated balance-sign convention, show source IDs and arithmetic, and reconcile total paid, total allocated and balances.” “Audit every original ID, amount, participant exclusion, remainder and person total against MY INPUT. Report exact discrepancies and unresolved questions. Do not compare with the guide’s example totals or alter rules to make totals fit.” “Return the full reviewed ledger, rules and a checklist for source completeness, participants, rounding and signs. Keep agreement and settlement as separate human decisions; no payments or messages.” Check every row and person total yourself. Save the final text as my-expense-review.txt, reopen it and keep it with the original source. If you request CSV separately, save only plain data and preserve signed numeric amounts and text IDs during any optional test-sheet import. These personal checks replace the worked-example checks. The original advanced extension is sample-only unless adapted to your actual people, currency and agreed rule.
Your own-input prompt
Help me reconcile a simple shared-expense list from MY INPUT. First extract every expense ID, exact amount/currency, payer and explicit participant list. Restate the supplied equal-within-item allocation rule, agreed remainder order and balance-sign convention. A known payer can be excluded from that item’s participants. Check duplicate IDs, unknown people, empty/repeated participant lists, negative or missing amounts, mixed currencies and more than two decimal places; do not silently fix them. Flag unapproved rules or uncertainty about source completeness and pause calculation until resolved. Do not invent what would be fair. Stop after extraction for my review. Return written analysis only; do not access accounts, send messages, propose transfers, move money or mark balances agreed or settled.
MY INPUT: PEOPLE, CURRENCY AND EXPENSES
[Use distinct neutral adult labels. Supply one two-decimal currency, unique expense IDs, exact already-paid amounts, payer and included participants for every item. State whether this is complete and identify any missing costs, refunds or unresolved records.]
MY INPUT: AGREED ALLOCATION RULES
[Confirm equal division within each item’s listed participants, or state that a different rule needs review. Supply the explicitly agreed order for assigning leftover minor units, skipping excluded people, and define your balance-sign convention. Do not invent agreement to fill these fields.]
Check your own result
Every actual expense ID is validated and allocated once; payer and participant lists stay separate and excluded people receive no share of that item.
Every item’s allocations sum exactly to its own amount under your supplied currency and agreed remainder rule. Different rules or missing approval remain unresolved.
Person paid totals and share totals derive from separate source columns. Balances use your explicitly stated sign convention and reconcile without changing the source.
Your saved result is checked against MY INPUT, not example names, counts or totals. Arithmetic balance does not prove completeness, participant agreement or settlement; no payment is made.
Show, then try
See the shape of the result.
This input is synthetic. The output is a written illustration of the intended result, not a verified AI-tool run.
01 / The starting point
Synthetic input
SYNTHETIC SHARED-OUTING RECORD
Three fictional adults: Avery, Blair and Casey. Currency: GBP. The six lines are the complete already-paid expense list; no taxes, fees, tips, refunds or other costs are missing. No bank, card or contact details are supplied.
AGREED ALLOCATION RULES
1. Split each expense equally only among its listed participants. A payer can pay for an item without being one of its participants. A person excluded from an item owes zero for that item.
2. Work in whole pennies. Divide the item's pennies by its participant count, give each person the integer share, then allocate any remaining pennies one at a time in this fixed agreed order: Avery, Blair, Casey, skipping people not listed for that item. This is the fixture's agreed tie-break rule, not a universal rule for groups.
3. Allocate each expense separately before adding a person's shares. Do not round an overall one-third share of the trip total or charge everyone for everything.
4. Balance means amount paid minus allocated share. Positive means paid more than the agreed share; negative means paid less. This is a review ledger, not a payment instruction. Everyone must check the inputs and rules before using a real result.
5. Reject duplicate expense IDs, an unknown payer, no listed participants, repeated participant names, a negative or missing amount, mixed currencies, or an amount with more than two decimal places. Do not silently repair these cases.
COMPLETE SOURCE
E01 | Group train tickets | paid by Avery | GBP 30.00 | participants: Avery, Blair, Casey
E02 | Museum tickets | paid by Blair | GBP 24.00 | participants: Avery, Blair
E03 | Shared lunch | paid by Casey | GBP 45.00 | participants: Avery, Blair, Casey
E04 | Parking for Blair and Casey | paid by Avery | GBP 9.00 | participants: Blair, Casey
E05 | Shared snacks | paid by Blair | GBP 10.00 | participants: Avery, Blair, Casey
E06 | Casey souvenir | paid by Casey | GBP 12.00 | participants: Casey
No AI workflow or payment has been performed. All names and amounts are fictional.
Copies text only. For image or file tasks, use the source files named in the steps too.
RECONCILED ILLUSTRATIVE LEDGER
Total paid: GBP 130.00. Total allocated: GBP 130.00.
Each row lists Avery / Blair / Casey shares:
E01: 10.00 / 10.00 / 10.00 = 30.00
E02: 12.00 / 12.00 / 0.00 = 24.00
E03: 15.00 / 15.00 / 15.00 = 45.00
E04: 0.00 / 4.50 / 4.50 = 9.00. Avery paid, but only Blair and Casey participated.
E05: 3.34 / 3.33 / 3.33 = 10.00. The one leftover penny goes to Avery under the supplied order.
E06: 0.00 / 0.00 / 12.00 = 12.00. Casey's own item creates no share for the others.
Participant | paid | allocated share | paid minus share
Avery | 39.00 | 40.34 | -1.34
Blair | 34.00 | 44.83 | -10.83
Casey | 57.00 | 44.83 | 12.17
Balances sum to zero. The two negative balances total GBP 12.17, equal to the positive balance. Positive is paid more than allocated; negative is paid less. These are authored expected results for the synthetic rules. The two CSVs contain static values, not live formulas. Review real inputs and obtain everyone's agreement separately; no payment route is provided.
These checks use the worked example and its particular facts. With your own material, check your actual names, numbers, sources and rules; do not reuse the sample’s values as the answer.
Unsaved checks stay in this tab and reset when you reload. If you choose Save guide, its current example checkmarks are kept in this browser when storage is available.
!
A few things to watch for.
Dividing the trip total equally when participants differ by expense, or confusing the payer with the beneficiaries.
Rounding each third independently and losing a penny, or changing the tie-break order without agreement.
Treating a balanced ledger as proof that the source is complete or that everyone agreed to reimbursements.
What has been checked?
Documentation
No dated source review recorded.
Included files
Scoped checks passed on 2026-10-14. Fixture checks. See scope below.
AI workflow
Not run in a recorded consumer-AI workflow.
Time savings
Time savings not measured
Read the scope and evidence
shared-expense-reconciliation.csv
2026-10-14 · semantic fixture · passed
Exact six-item synthetic allocation and zero-sum balance checks with whole pennies. Plain-value CSV authoring, not a consumer spreadsheet or AI workflow run.
Six unique expense IDs; one currency and three fictional adults
Each item sums exactly to its supplied pennies
Payer exclusion and individual purchase are preserved
Remainder penny goes to Avery under the agreed order
Total paid and allocated GBP 130.00; balances sum to zero
Environment: Node integer arithmetic and artifact-tool typed cells; local fixture authoring
Exact six-item synthetic allocation and zero-sum balance checks with whole pennies. Plain-value CSV authoring, not a consumer spreadsheet or AI workflow run.
Six unique expense IDs; one currency and three fictional adults
Each item sums exactly to its supplied pennies
Payer exclusion and individual purchase are preserved
Remainder penny goes to Avery under the agreed order
Total paid and allocated GBP 130.00; balances sum to zero
Environment: Node integer arithmetic and artifact-tool typed cells; local fixture authoring
Exact six-item synthetic allocation and zero-sum balance checks with whole pennies. Plain-value CSV authoring, not a consumer spreadsheet or AI workflow run.
Six unique expense IDs; one currency and three fictional adults
Each item sums exactly to its supplied pennies
Payer exclusion and individual purchase are preserved
Remainder penny goes to Avery under the agreed order
Total paid and allocated GBP 130.00; balances sum to zero
Environment: Node integer arithmetic and artifact-tool typed cells; local fixture authoring