CONTRIBUTION RULE COMPARISON
Compare equal dollars with equal income burden before calling a split fair
This calculator holds one shared monthly savings target constant and compares three allocation rules: equal cash, income-proportional cash, and the partners' current proposal. It quantifies income burden without treating income as the only relevant contribution to a household or relationship.
| Allocation rule | Partner A USD | Partner B USD | A share % | A burden % | B burden % |
|---|
Detailed calculation process
Formula, declared symbols, substitutions, intermediate results, and reconciliation
sA = IA/(IA+IB); pA = GsA; pB = G−pA; eA=eB=G/2; qA=xA/IA; qB=xB/IB
The target remains fixed while the model changes only the allocation rule. Income-proportional cash gives each partner the same percentage burden; equal cash gives each the same dollar amount; the proposal is measured without being endorsed.
| Symbol | Meaning | Default | Unit |
|---|---|---|---|
| G | Combined monthly savings target | 1,000 | USD/month |
| IA, IB | Partner monthly income bases | 5,000; 3,000 | USD/month |
| xA, xB | Proposed contributions | 700; 300 | USD/month |
| sA | Partner A income share | calculated | dimensionless |
| qA, qB | Contribution-to-income burdens | calculated | percent |
Default substitution: combined income = $8,000 and sA = 5,000/8,000 = 62.5%. The proportional split is $625/$375, equal cash is $500/$500, and the proposal burdens income at 14% versus 10%.
Five comparison steps
Define the contribution rule before negotiating the amount
- Agree on one monthly goal and verify that it is affordable at the household level.
- Choose a matching income basis and period for both partners; do not mix gross annual income with net monthly cash.
- Enter the current proposal exactly, including a zero contribution where that is the real agreement.
- Compare equal cash, proportional cash, and income burdens without labeling any output morally correct.
- Add non-income context in writing, then record the rule and a review date if the partners agree.
Five allocation fundamentals
Recognize what each comparison rule preserves
Fixed target
The monthly savings amount held constant across all allocations.
Equal cash
The same dollar transfer from each partner, regardless of income.
Income proportion
Cash allocated by each partner's share of combined entered income.
Income burden
A contribution divided by that partner's own income basis.
Context adjustment
An agreed change based on factors the numerical comparison cannot represent.
Three fairness deep dives
Know where an apparently neutral rule carries hidden choices
Income definition
Gross, take-home, variable, and restricted income create different proportions. The chosen basis must be visible and consistently updated.
Non-cash contribution
Caregiving, household work, relocation, and career sacrifices can be economically important but do not appear in the income ratio.
Separate obligations
Debt, support duties, disability costs, and emergency needs change available cash. A burden percentage based only on income may still be unsustainable.
Allocation evidence
Freeze the income basis and the reasons for any adjustment
Retain pay records or agreed income estimates, the shared goal budget, proposed contributions, separate mandatory obligations, review dates, and a written explanation for deviations from equal or proportional cash. Recalculate after a material income change.
Comparison limitations
What a contribution ratio cannot measure
- Income is the only capacity measure in the proportional rule.
- Taxes, debt, wealth, dependents, benefits, and income volatility are excluded.
- Unpaid labor and non-cash support are not assigned monetary values.
- The calculator does not establish ownership shares in the resulting savings.
- No output determines fairness, consent, legal duty, or relationship quality.
Contribution glossary
Six terms for discussing a shared savings split
- Allocation rule
- The declared method used to divide one fixed target.
- Equal cash
- A fifty-fifty dollar split.
- Income share
- One partner's entered income divided by combined income.
- Income burden
- Contribution as a percentage of the contributor's income.
- Burden gap
- The absolute percentage-point difference between burdens.
- Review trigger
- An agreed event that reopens the allocation, such as an income change.
Two allocation cases
Similar incomes and asymmetric caregiving
Comparable take-home pay
Two partners with similar income choose equal cash because it is simple and produces nearly equal burdens, then schedule a review after annual raises.
One reduced work schedule
Income proportionality suggests a lower cash amount for the partner providing more unpaid care. The couple documents that the formula informs, but does not value, the caregiving contribution.
Important note
Pause when money discussions involve pressure, secrecy, or control
A numerical split requires informed agreement and access to accurate information. If a partner cannot safely decline, review records, or access essential money, do not use this calculator to legitimize the arrangement; seek appropriate confidential support.
Contribution-rule questions
Frequently asked questions
Does income-proportional mean fair?
Not universally. It equalizes the percentage of entered income assigned to saving, but ignores unpaid work, debt, assets, caregiving, and personal obligations.
Why must the proposed contributions equal the target?
The comparison is about allocation of one fixed monthly goal. A proposed shortfall would mix fairness analysis with goal feasibility.
What is the burden gap?
It is the absolute percentage-point difference between each proposed contribution divided by that partner’s entered monthly income.
Should gross or take-home income be entered?
Use one agreed basis for both partners. Take-home cash may better match transfer capacity, but the calculator does not prescribe a definition.
Can one partner contribute zero?
Yes, if the other proposed contribution still equals the target. The page will show the resulting income-burden difference without judging the agreement.
Does this create a contract?
No. It is a transparent comparison worksheet and cannot create, interpret, or enforce a financial or relationship agreement.
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