Timing changes the annuity
Beginning-of-month deposits would grow for one extra period; this model uses month end.
Shared planning
Couple Budget Goal Calculator
Solve the combined monthly deposit for a dated shared goal, split it by an entered share, and compare an existing plan.
DATED SHARED GOAL
Solve the deposit first, then assign the agreed contribution shares
This page compounds the current balance, solves a month-end deposit annuity for a target date, divides the requirement by an entered share, and forward-checks the partners’ existing planned deposits.
Required combined deposit-
Required Partner A deposit-
Required Partner B deposit-
Projected with entered plan-
Target minus projection-
Future value of current savings-
DATED SHARED GOAL
Goal deposit solution and forward-check ledger
A yield is uncertain and a target date can move. Save a zero-yield case and a shorter-horizon case before treating the required deposits as affordable.
| Layer | Monthly or current amount | Months or share | Yield or role | Future value |
|---|
DETAILED CALCULATION PROCESS
Formula, symbols, defaults, conversions, live substitution, and reconciliation
1. General symbolic formula
r=y/12; FV_0=S(1+r)^n; a=((1+r)^n−1)/r; D=(T−FV_0)/a; D_A=sD
2. Calculation logic
Convert annual yield to a monthly decimal, compound the current balance, compute the ordinary-annuity factor for month-end deposits, solve the combined deposit, and split it by Partner A’s entered percentage.
3–4. Symbol names, meanings, and units
| Symbol | Meaning | Unit |
|---|---|---|
| T | Goal target | USD |
| S | Current saved amount | USD |
| n | Deposit cycles | months |
| y | Annual yield | percent/year |
| r | Monthly decimal yield | per month |
| a | Month-end annuity factor | — |
| D | Required combined deposit | USD/month |
| s | Partner A share | decimal |
5. Default inputs
- Shared goal target: 15000
- Current saved amount: 3000
- Months to goal date: 24
- Estimated annual yield (%): 3
- Partner A share (%): 60
- Partner A planned monthly deposit: 280
- Partner B planned monthly deposit: 180
6. Percentage and unit conversions
- Annual percent becomes a decimal and is divided by 12.
- Deposits are assumed at each month end.
- At zero yield, the annuity factor equals the month count.
7–9. Live substitution, intermediate results, and final result
10. Reverse or reasonableness check
HOW TO USE
Build a goal plan that survives a forward check
- Define the amount and exact month count.
- Use only the current balance dedicated to this goal.
- Enter a conservative yield and save a zero-yield version.
- Choose the contribution share after solving the combined requirement.
- Compare the entered plan with the target and schedule a review.
SUBJECT FUNDAMENTALS
Five parts of a dated savings goal
- Target
- Required future balance.
- Present balance
- Money already assigned to the goal.
- Deposit cycle
- One month-end contribution.
- Annuity factor
- Future accumulation of equal monthly deposits.
- Plan gap
- Target minus the entered plan projection.
MODEL AND FORMULA
Solve total accumulation before splitting contributions
r=y/12; FV_0=S(1+r)^n; a=((1+r)^n−1)/r; D=(T−FV_0)/a; D_A=sD
Convert annual yield to a monthly decimal, compound the current balance, compute the ordinary-annuity factor for month-end deposits, solve the combined deposit, and split it by Partner A’s entered percentage.
DEEPER DECISION ANALYSIS
Three sensitivities hidden in one monthly number
Yield is not guaranteed
A quoted or historical rate can change, and nondeposit products can lose value.
Share and affordability differ
A percentage rule can produce a correct split that one partner cannot sustain.
WORKED DECISION CASES
Two goal-plan cases worth saving
Zero-yield floor
Set yield to zero to see the deposit requirement without assumed growth.
Contribution interruption
Shorten the effective deposit count to represent missed months, then compare the increase.
TECHNICAL LANGUAGE
Goal-accumulation glossary
- Future value
- Balance after modeled growth and deposits.
- Monthly yield
- Annual estimate divided into monthly decimal periods.
- Ordinary annuity
- Equal deposits made at period end.
- Contribution share
- Entered fraction of the combined requirement.
- Projection
- Calculated future balance under assumptions.
- Shortfall
- Positive amount target exceeds projection.
EVIDENCE AND DATA LINEAGE
Preserve the product and timing assumptions
Keep the goal statement, current balance record, deposit dates, rate source and date, fee information, account type, contribution-share agreement, and planned review date. Recalculate after a missed deposit or rate change.
LIMITS AND EXCLUSIONS
Limits of the goal projection
- Yield is constant and compounded monthly; actual results may vary.
- Taxes, inflation, fees, and market losses are omitted unless already reflected.
- The model assumes equal month-end deposits.
- It does not determine ownership or withdrawal rights.
- It is not investment, tax, legal, or relationship advice.
RELIABLE SOURCES
Primary and authoritative planning references
FREQUENTLY ASKED QUESTIONS
Questions about a joint goal deposit
Why can required deposit be zero?
The current balance may already grow to or beyond the target under the entered assumptions.
Are deposits made at month start?
No, the formula assumes month-end deposits.
Is the yield guaranteed?
No. It is an input for scenario analysis.
Must the share match income?
No. It is a separate agreement entered directly.
Why compare planned deposits?
It shows whether the current behavior matches the solved requirement.
Does a contribution create ownership?
This calculator does not determine legal or beneficial ownership.
IMPORTANT NOTE
A projection is not a promise
Verify account terms, fees, risks, insurance category, taxes, ownership, and withdrawal rules. The calculator models arithmetic under entered assumptions and does not recommend a product or contribution agreement.
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