Scenario builder
Model any financial decision as two or three side-by-side scenarios. Rent vs buy, save vs invest, pay debt vs invest — define the assumptions and watch the trajectories diverge.
Calculator compareLogic updated April 2026
This calculator is a general-purpose what-if engine: you define multiple scenarios with starting amounts, monthly changes, growth rates, and one-time costs, and it projects each year-by-year over a chosen horizon. Useful for any scenario where you want to see how different starting positions and ongoing changes evolve into different end-of-horizon outcomes.
How this is calculated
Formula
Year 0: balance = initialAmount − oneTimeCosts ; each year: balance += monthlyChange × 12 ; balance × (1 + annualGrowthRate / 100)Step-by-step
- Start each scenario at year 0 with (initial amount minus any one-time costs)
- Each year, add 12 × monthly change to the balance — positive for contributions, negative for withdrawals
- Apply the annual growth rate (compounded once per year)
- Track year-end values for each scenario across the projection horizon
- Identify the best and worst scenarios at end of horizon
- Report the crossover year — the first year-boundary where the leader changes from year 0
- Rounding mode
- ROUND_HALF_UP
- Precision
- 20-digit internal precision (Decimal.js), rounded to 2 decimal places for display
- Logic last reviewed
Assumptions & limitations
What this calculator assumes
- Growth rates and monthly changes are held constant for the projection horizon
- One-time costs are deducted from the starting balance before any growth or contribution
- Each year's contributions (monthly change × 12) are added to the balance before growth is applied
- Compounding cadence is annual — finer cadence does not change the qualitative comparison
- Negative initial amounts represent debts; positive amounts represent assets
- Negative monthly changes represent payments / withdrawals; positive amounts represent contributions
- Crossover year is the first year-boundary where the leader of pairwise final-value ordering changes from year 0
- No taxes, transaction costs, or inflation are modelled
What this calculator doesn’t account for
- Doesn't model varying growth rates over the horizon
- Doesn't include taxes on growth or contributions
- Doesn't factor in inflation — outputs are nominal
- Doesn't model variable monthly contributions or seasonal patterns
- Doesn't include risk or volatility — deterministic projections only
Worked example
Compare three financial paths over 25 years: A) save $1,000/month at 7% growth from $0; B) pay extra $1,000/month into a $400,000 mortgage at 6% (negative growth on debt); C) save $500/month at 5% from $50,000 starting position.
| Input | Value |
|---|---|
| Scenario A | $0 + $1,000/month / 7% growth |
| Scenario B | −$400,000 + $1,000/month / 6% (debt unwind) |
| Scenario C | $50,000 + $500/month / 5% |
| Horizon | 25 years |
Scenario A: ~$810k — Scenario B: ~$0 (debt cleared around year 22) — Scenario C: ~$472k. Best is A by $338k.
Scenario A starts at zero but $1k/month at 7% over 25 years is a powerful accumulator. Scenario B uses the same monthly cash flow but to retire debt at a lower rate — by year 22 the debt is cleared, but the cash flow stops creating wealth at that point. Scenario C has a head start but lower contributions and lower return. The 25-year horizon favours aggressive accumulation; over a 10-year horizon, B and C might compete more closely.
Frequently asked questions
What is a financial scenario builder?
A general-purpose tool for projecting any starting financial state under any pattern of contributions and growth. It's domain-agnostic — works for savings, investments, debt repayment, education savings, or any combination. The strength is flexibility: you can mix asset and debt scenarios in the same comparison and see how they evolve side-by-side over a long horizon.
How many scenarios should I compare?
Three to four is the practical sweet spot. Two scenarios miss the chance to find a middle option; five or more makes the chart unreadable and the analysis paralysed. Common useful triples: aggressive vs moderate vs conservative; current plan vs proposed change vs alternative; high-risk-high-reward vs low-risk-steady vs balanced.
What inputs matter most?
Annual growth rate dominates over long horizons — small differences (1–2 percentage points) compound into 30–60% gaps in final value over 25+ years. Monthly contribution dominates over short horizons (under 10 years). Starting amount matters most for shorter horizons and lower contribution rates. Use the calculator to test sensitivity by varying one input at a time.
How do I interpret the comparison?
Look at three things: (1) the final year balance for each scenario — the headline number; (2) the crossover year — when leadership changes, signalling a 'short-term winner becomes long-term loser' pattern; (3) the year-by-year curve — does one scenario take an early lead and hold it, or do they swap order multiple times? Stable winners are easier to commit to; volatile rankings suggest the comparison is sensitive to inputs and deserves stress-testing.
Can I model debt repayment in this calculator?
Yes — use a negative initial amount (the debt balance) with a negative monthly change (payments) and a positive growth rate (interest rate). The balance becomes less negative over time as payments exceed interest. This is a simplified treatment versus the dedicated loan calculators (no amortisation), but it's useful for quick what-ifs comparing debt repayment against alternative uses of the cash flow.
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