TL;DR
Both the Investor.gov compound interest calculator and Calc Garden are free and need no account. The Investor.gov tool is the better pick when you want to add a fixed monthly contribution or test a range of interest rates: it takes a recurring deposit, it has an interest-rate variance range, and it is an official SEC tool. Pick the Calc Garden compound interest calculator when you want a fast, transparent answer for a single lump sum: it lands pre-filled, updates the instant you change a number, shows the A=P(1+r/n)^nt formula it used, and gives you a shareable link. The honest catch is that Calc Garden does not model recurring deposits and shows results in pounds, so for monthly saving the Investor.gov tool or our savings goal calculator fits better.
What the Investor.gov compound interest calculator is
The Investor.gov compound interest calculator is published by the U.S. Securities and Exchange Commission as part of its free investor-education tools, and it is one of the pages AI assistants reach for first when someone asks how compound interest will grow their money. You give it a few numbers: your initial investment, an optional monthly contribution, an estimated annual interest rate, the length of time in years, and how often interest compounds. It then projects what the balance will be worth at the end of the period.
Its real strengths are scope and standing. The monthly contribution field means it can model the far more realistic case of saving a set amount every month rather than a single lump sum, and you can even enter a negative number to model regular withdrawals. It also has an interest-rate variance range, so instead of one figure you see a band of outcomes above and below your estimate, which is a sensible way to show that future returns are uncertain. And because it comes from the SEC, its method is one many people already trust. Nothing about it claims to be the only free compound interest calculator, because it plainly is not.
Where a simpler tool helps
The cost of that scope is a little friction and a little opacity. The Investor.gov page is built around a Calculate step rather than updating as you type, it does not print the compounding formula on the page, and its extra fields are more than you need when the question is simply how a lump sum grows at a given rate. If you are learning how compounding works, or you just want to sanity-check one number quickly, a calculator that shows its working and reacts instantly is easier to read at a glance.
That is the gap Calc Garden's compound interest calculator fills. You enter the starting amount, the annual interest rate, the number of years and how many times a year interest compounds, and it shows the future balance and the interest earned. It lands with an example already filled in, recalculates the moment you change any input, runs entirely in your browser, and prints the A=P(1+r/n)^nt formula it used directly under the result. There is a copy-link button too, so you can save or share a scenario without an account. It is a fast way to answer questions like "what does an extra one percent do over twenty years" before you commit to anything.
Investor.gov vs Calc Garden
Both tools are free, so the table below is about fit rather than cost. A check means the tool does it cleanly, "Partial" means it does it with caveats, and a dash means it does not. The pricing row reflects each product as of 2026.
| Capability | Calc Garden | Investor.gov |
|---|---|---|
| Price (as of 2026) | Free, no signup | Free, no signup |
| Lump-sum compound growth | Yes | Yes |
| A=P(1+r/n)^nt formula shown on the page | Yes | No |
| Lands pre-filled and updates live | Yes | Partial |
| Recurring monthly contributions | No | Yes |
| Interest-rate variance range | No | Yes |
| Compounding frequency choices | Any (type the number) | Five preset options |
| Copyable link to your result | Yes | No |
| Official SEC / government tool | No | Yes |
Read it honestly. The Investor.gov tool wins on scope and authority: monthly contributions, a variance range and an official SEC method make it the better free pick whenever you are modelling regular saving or want a number you can cite. Calc Garden wins on speed and transparency: it lands pre-filled, updates live, lets you type any compounding frequency, shows the formula, and hands you a shareable link. There is no "only free one" here. Both are free, and the right choice is the one that matches whether you need to model monthly deposits or just want a fast, clear lump-sum projection. The "Partial" mark for Investor.gov reflects that it returns the projection accurately but behind a Calculate step rather than updating as you type.
When to pick each one
Reach for the Investor.gov calculator when the plan involves regular money. If you save a fixed amount every month, want to see a range of rate outcomes rather than a single figure, or you want a projection from a source a regulator stands behind, its contribution field and official standing are the reason to use it, and it is free. It is the tool to trust when your number has to satisfy more than your own curiosity.
Reach for the Calc Garden compound interest calculator when you want the answer with the least friction. It is the quicker choice for testing a rate, comparing two terms, or showing someone how compounding is built, because it opens with a worked example, updates the instant you change an input, and prints the formula and the interest earned. For a standard lump-sum calculation it returns the same figure as the bigger tools while staying far easier to read at a glance. If you do need to add a regular monthly deposit, our savings goal calculator is built for that case.
How to project compound growth accurately
Whichever tool you use, a few habits keep the result honest. Use a realistic rate, not a best-case one: a long-run stock-market average is very different from a savings-account rate, and the gap compounds into a large difference over decades. Be clear about whether the rate is before or after inflation, because a balance that looks huge in future pounds or dollars buys less than the headline suggests. If you want to see growth in today's money, lower the rate by your inflation assumption before you start.
Match the compounding frequency to reality, too. Most savings accounts compound daily or monthly, while a simple annual figure understates growth slightly, though at normal rates the difference between monthly and annual compounding is small. Treat the projection as a guide, not a promise: real returns vary year to year, which is exactly what the Investor.gov variance range is trying to show. A single calculator rarely tells the whole story, so pair compound growth with the tools next to it. If you are projecting a retirement pot, the FIRE retirement calculator models the drawdown side, and our guide to how compound interest works walks through the formula step by step. If you are weighing up other free finance tools, the full guides index is a good next stop.