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Compound Interest Calculator
Project a lump sum and monthly contributions — with inflation-adjusted real value and a savings goal. Accurate, instant and free — for US · India.
What these mean:
What these mean:
Your $10,000 plus $500/month grows to this in 10 years at 7%.
Growth over time
10 yrsHow compound interest is calculated
Compound interest grows a principal by reinvesting earned interest each period, so that interest itself earns interest. This calculator supports lump-sum growth, regular contributions, selectable compounding frequency, deposit timing (ordinary vs. annuity-due), and an optional inflation-adjusted real value.
Lump-sum growth
The classic compound interest formula
P = principal · r = annual rate (decimal) · n = compounds/year · t = years
The effective annual rate (EAR) is (1 + r/n)^n − 1, which lets you compare daily, monthly, and annual compounding on equal footing. Daily compounding (n = 365) produces slightly more than monthly (n = 12), but the gap is modest — roughly $39 on $10,000 over 10 years at 7%.
With regular contributions
Future value of an annuity added on top
i = rate per contribution period · N = # contributions · timing = (1+i) start-of-period, 1 end-of-period
The timing factor matters: deposits at the start of each period (annuity-due) earn one extra period of interest vs. deposits at the end (ordinary annuity). Most savings accounts use ordinary (end-of-period); this calculator lets you toggle between them.
When compounding ≠ contribution frequency
Convert to an effective per-contribution rate first
Most basic calculators silently force compounding and contribution frequency to be equal. This calculator keeps them independent — the key correctness lever identified by thecalculatorsite.com, the feature-richest incumbent. When the frequencies differ, convert the nominal annual rate to an effective rate per contribution period before applying the annuity formula:
n = compounds/year · f = contributions/year
This ensures the accumulated value is mathematically consistent regardless of whether you compound daily but contribute monthly, or vice versa.
Inflation-adjusted real value
What your future sum is worth in today's purchasing power
e.g. $106,639 nominal ÷ (1.03)^10 ≈ $79,349 real (3% infl, 10 yr)
Inflation silently erodes purchasing power. A savings balance that looks impressive in nominal terms may buy significantly less in real terms. At 3% inflation over 10 years, a dollar today is worth only about $0.74 in future money.
A full compound interest calculation, step by step
Initial lump sum $10,000; monthly contribution $500; annual rate 7%; compounding and contribution both monthly (i = 0.07/12 ≈ 0.58333%, N = 120 periods).
- 1Lump-sum growth: A = 10,000 × (1.0058333)^120 ≈ 10,000 × 2.00966 = $20,097.
- 2Contributions (ordinary annuity): FVcontrib = 500 × [((1.0058333)^120 − 1) / 0.0058333] ≈ 500 × 173.0848 = $86,542.
- 3Combined future value: $20,097 + $86,542 = $106,639 nominal.
- 4Total invested: $10,000 principal + (120 × $500) = $10,000 + $60,000 = $70,000. Interest earned: $106,639 − $70,000 = $36,639.
- 5Inflation-adjusted real value (3% p.a.): $106,639 ÷ (1.03)^10 = $106,639 ÷ 1.3439 ≈ $79,349in today's purchasing power.
Projection assumptions
Frequently asked questions
The lump-sum compound interest formula is A = P × (1 + r/n)^(n×t), where P is the principal, r is the annual interest rate (decimal), n is the number of compounding periods per year, and t is the number of years. For example, $10,000 at 7% compounded monthly for 10 years: A = 10,000 × (1 + 0.07/12)^(12×10) ≈ $20,097. This formula is confirmed by SEC investor.gov and standard finance references.
Regular contributions are added using the future value of an annuity formula: FV_contrib = PMT × [((1+i)^N − 1) / i] × timing_factor, where i is the interest rate per contribution period, N is the total number of contributions, and timing_factor is (1+i) for start-of-period deposits (annuity-due) or 1 for end-of-period deposits (ordinary annuity). The total future value is the lump-sum result plus FV_contrib. For example, $500/month ordinary contributions at 7% monthly for 10 years grow to approximately $86,542, bringing the combined total (with a $10,000 principal) to about $106,639.
Daily compounding (n=365) produces slightly more than monthly compounding (n=12) because interest is reinvested more frequently, but the difference is modest in practice. For $10,000 at 7% over 10 years: monthly compounding gives about $20,097; daily compounding gives about $20,136 — a difference of roughly $39 (0.2%). The more meaningful lever is the interest rate and time horizon, not the compounding frequency. The effective annual rate (EAR) formula EAR = (1 + r/n)^n − 1 lets you compare different compounding frequencies on equal footing.
The inflation-adjusted real value strips out the effect of rising prices, showing what your future sum is worth in today's purchasing power. The formula is: real value = nominal FV / (1 + inflation rate)^t. For example, a nominal future value of $106,639 at 3% annual inflation over 10 years has a real value of approximately $79,349 — meaning it buys about as much in today's terms as $79,349 does now. This is not investment advice; real returns vary and inflation is a user-supplied assumption.
Method, assumptions & references
Methodology note: lump-sum formula A = P(1+r/n)^(nt) confirmed by SEC investor.gov and standard finance references. Annuity FV formula and timing-factor (ordinary vs. annuity-due) confirmed by CalculatorSoup. Compounding-frequency independence and deposit-timing treatment cross-checked against thecalculatorsite.com. Worked example ($106,639 nominal; $79,349 real at 3%) computed from first principles and confirmed to engine-exact precision. Rate and inflation are user inputs; no baked-in market data. All figures are projections, not investment advice; returns are not guaranteed.
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How we calculate this
Reviewed by Reckonist Editorial · Last reviewed 13 June 2026. Figures follow the methods and sources set out in our editorial standards.
This is a projection based on the figures you enter and assumes a constant rate of return; real returns vary and are not guaranteed. It is general information, not personal investment or tax advice.
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