Free calculator
Compound interest calculator
Estimate how a starting balance and optional monthly contributions grow under a nominal annual rate with the compounding frequency you choose. Use it as a quick check before you wire the same logic into Google Sheets or Excel—then copy the FV pattern below.
When to use this calculator
Top-of-funnel planning checks where a constant rate and regular contributions are enough—transparent and file-independent.
- Ballpark how savings might grow with monthly deposits and bank-style compounding (daily, monthly, quarterly, and so on).
- Compare the effect of compounding frequency holding the same nominal annual rate—useful when you explain APR-like ideas to teammates.
- Sanity-check a long horizon before you expand the model with variable returns, taxes, or cash flows in a spreadsheet.
- Teach the difference between total contributed and interest earned when someone is new to compounding.
We use one nominal annual rate compounded m times per year (daily uses 365 sub-periods). Your horizon is converted to a whole number of months. Each month we apply the same growth factor a spreadsheet would use when it turns a nominal rate into a monthly step.
Periodic rate from the nominal annual rate
With nominal annual rate r and m compounding periods per year, one month uses growth factor (1 + r/m)^(m/12). That lines up with (1 + r/m)^(m·t) for lump sums when t is in years.
Starting balance (lump sum)
Your initial amount compounds for N months using the monthly factor above. With no contributions, ending balance equals starting balance × g^N where g is that factor.
End-of-month contributions (annuity)
Each monthly payment is added at month end after interest for that month. With growth factor g and i = g − 1, the closed form is PV·g^N + PMT·((g^N − 1) / i) when i ≠ 0. When the rate is 0, ending balance is PV + PMT·N.
We round only for display; internal math uses floating point. If your sheet uses different day-count rules, bank fees, or beginning-of-period payments, expect small differences.
Nominal vs APY, simple interest, and FV signs are covered in the FAQ below.
Google Sheets & Excel (FV)
This page assumes end-of-month contributions. In Sheets or Excel, FV uses signed cash flows: pass negative numbers for money you invest today and each month so a positive result matches “future balance.” Replace cell references with yours.
=FV(annualRate/12, years*12, -monthlyPayment, -startingAmount, 0)Example layout: annualRate as 0.05 for 5%, years as 10, monthlyPayment and startingAmount as positive numbers in the sheet—negate them inside FV as shown. The last 0 means payments at end of each period, matching this calculator.
Frequently asked questions
What is compound interest?
Compound interest means interest earns interest. Each period, the rate applies to your current balance (including prior interest) and any contributions you model—so the balance increases faster than simple interest on the original principal alone.
Is the rate on this page APY or nominal?
We treat your input as a nominal annual rate with the compounding frequency you select. APY is an effective annual figure you might see on a savings product; you can relate APY to nominal when you know how often interest compounds, but this page does not ask for APY directly.
What if my contributions do not match the compounding frequency?
This calculator keeps monthly contributions and converts the nominal rate into a monthly growth factor that matches your compounding choice. That is a standard way to stay consistent when deposits are monthly but interest compounds daily or quarterly. If you need weekly deposits or beginning-of-period timing, rebuild the timeline in a sheet.
How is compound interest different from simple interest?
Simple interest grows only on the original principal each period. Compound interest applies to the current balance, so the interest portion can increase over time when the rate is positive.
Is 1% per month the same as 12% per year?
Only in a simple sense. 12 × 1% is twelve percentage points of simple interest on the original principal over a year. Compounding means that 1% each month applies to an increasing balance—so the effective annual result is usually greater than 12% when interest compounds monthly at 1% per month.
How do I match this page in Google Sheets or Excel?
Use =FV(annualRate/12, years*12, -monthlyPayment, -startingAmount, 0) with the same sign convention as the formula card—negate outgoing cash. If you only have a lump sum, set monthlyPayment to 0. If your sheet models beginning-of-period payments, change the last argument to 1 instead of 0.
Can I use this for a 401(k) or retirement projection?
The math shape (balance plus periodic contributions at a constant rate) is similar to a toy retirement illustration, but real accounts have contribution limits, employer match rules, taxes, fees, and changing returns. Treat this page as a teaching tool, not a retirement plan.
Is this investment advice?
No. This page is a free educational calculator. It does not know your goals, time horizon, or risk tolerance—talk to a qualified professional when decisions matter.