Purposeful FIRE📋Budget Surplus & Investment Potential
Enter your monthly income and expenses by category to find exactly how much you have left to invest each month, your savings rate, and what that surplus compounds to over 10, 20, and 30 years. The investable surplus is the single most important number in a FIRE plan.
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What Is Budget Surplus & Investment Potential?
Every FIRE plan ultimately rests on one number: the monthly investable surplus. Your FIRE number divided by your monthly surplus tells you how many months of investing it takes to reach financial independence. A household earning $9,000/month after tax and spending $7,000 has a $2,000 surplus and a 22% savings rate. A household earning the same $9,000 and spending $5,500 has a $3,500 surplus and a 39% savings rate. The second household reaches FIRE in roughly half the time.
This calculator does two things: it makes the surplus explicit by categorizing all income and expenses, and it shows what that surplus becomes over 10, 20, and 30 years at a given investment return. The compound growth projection is often the most motivating part, because $2,000/month at 7% becomes over $1.2 million in 20 years — a number that makes the connection between current spending decisions and long-term wealth viscerally concrete.
How This Calculator Works
Monthly surplus = total income (take-home pay + other income) minus total expenses (sum of all categories). Annual surplus = surplus × 12. Savings rate = surplus / total income × 100. The compound growth projection uses the future value of an annuity formula: surplus × ((1 + monthly rate)^n - 1) / monthly rate, where n is the number of months.
Personal Considerations
Seeing the full budget on one screen makes tradeoffs explicit in a way that mental accounting does not. Most people have a rough sense of big expenses (rent, car) but underestimate the cumulative weight of smaller recurring items. The average American household has $200-400/month in subscription and membership fees they no longer actively use. $300/month invested at 7% for 20 years is $184,000. Seeing that specific comparison is what converts abstract advice ('cut subscriptions') into motivated action.
The compound growth projections are intentionally shown over multiple horizons because people differ in what time frame motivates them. Some people are most motivated by the 10-year number (it feels close enough to be real). Others are unmoved by 10 years but galvanized by the 30-year figure. Showing all three allows each person to find their motivating horizon.
If what you're feeling goes beyond what a calculator can help with, licensed clinicians are available at SanaNetwork.com, a referral network founded by this site's founder, Dr. Yoendry Torres.
Frequently Asked Questions
Net (take-home) income in this calculator. This avoids the complexity of separately modeling taxes as an expense. If you want to verify your effective tax rate or model pre-tax contributions, use the savings rate calculator, which handles gross-to-net conversion.
Divide annual amounts by 12 and add them to the appropriate monthly category, or add them to 'other.' This gives a more accurate monthly average than only capturing the months when you actually pay. A $1,200 annual car insurance premium is effectively $100/month.
A negative surplus means spending exceeds income — which is only sustainable through debt accumulation. The fastest improvements come from the highest absolute-dollar categories, usually housing and transportation. Eliminating or downsizing a car payment often saves $400-700/month instantly. Moving to a lower-cost living situation is the highest-impact single change for most households.
The Savings Rate Calculator is focused on computing the FIRE timeline from your savings rate: how many years to financial independence. This Budget Surplus calculator is focused on finding your savings rate from a detailed budget: what exactly is your surplus, and what does each category cost? They are complementary — use this one first to find your surplus, then plug the result into the Savings Rate Calculator to find your timeline.