Compound interest & contributions
Explore the effects of time and regular contributions.
See examples and explanations ↓A nominal, pre-tax simulation. Taxes, fees, and inflation are excluded. Returns are not guaranteed.
Simulation results
Enter your assumptions, then select Calculate.
JSON
Up to 1 MiB (1,048,576 bytes) and 1–3 scenarios. Files stay in your browser. Current work is preserved until you confirm.
What this tool explores
Explore a lump sum on its own or with regular contributions. Change the duration or assumed return to see how principal, added contributions and gains contribute to the final total.
Step by step
- Enter the initial lump sum as principal and each later payment as the contribution per period. Use zero when there are no additional payments.
- Choose the frequency and start or end of period. A contribution of 100 monthly means 12 payments per year; 100 every three months means four. Their annual contributions differ.
- Enter the duration and effective annual return, then calculate. Add a scenario to compare a different assumption.
What to look for in these results
Read contributed amounts alongside portfolio value in the yearly table. Growth caused by extra payments is not investment gain. With negative returns, portfolio value can fall below total contributions.
Questions about this tool
Why not divide the annual return by 12?
The input is an effective annual return. The monthly rate compounds over 12 months to that annual return. The annual dividend yield in separate-dividend mode is a different, nominal payout assumption.
What changes with start versus end of period?
A start-of-period payment enters before that period’s return is applied. It participates in positive returns for longer, but also has longer exposure when the assumed return is negative.
Calculation method
The annual return is an effective annual rate. Monthly return is (1 + annual return)^(1/12) − 1. Contributions occur at the beginning or end of each 1-, 3-, or 12-month period. Total-return mode already includes dividends, so no additional dividend is added.
Separate-dividend mode assumes a monthly payout of the opening value after any start-of-month contribution × annual yield ÷ 12. After price growth, the dividend is reinvested or held as non-interest-bearing cash, followed by any end-of-month contribution. This does not model real securities’ payout calendars. Values are rounded only for display.
Reading the results
Total gain is portfolio value plus cumulative cash dividends minus initial principal and subsequent contributions. Cash dividends are separate from portfolio value. The goal tool solves for the contribution per period under your assumptions; the target includes cumulative cash dividends.
Limitations and FAQ
Does this predict future returns?
No. This is a simulation with constant assumed returns and yields. It does not predict actual losses or volatility.
Are my inputs stored?
Inputs stay in this page’s memory and are lost when you leave. Download a JSON backup if needed. The operator cannot recover your file.
Calculation version: invest-1.0.0 · Source review date: 2026-10-05. Investor.gov (reference for compound-interest inputs; not a validation of ToffeeLab’s monthly dividend assumptions)