The same formula, run backwards
With monthly compounding and a deposit at the end of each month, a starting amount P and a monthly deposit C grow to P times (1 + r/12) to the 12t, plus C times that growth factor minus one, divided by r/12. The compound-interest calculator evaluates that expression. This page solves it for C. If the starting amount, grown alone, is already at least the goal, C is zero. If the rate is zero, C is the gap divided by the number of months. Otherwise C is the gap after the starting amount has grown, times the monthly rate, divided by the growth factor minus one. The deposit shown is that result rounded half-up to the cent, and the balance shown is what the compound function does with that rounded deposit.
The checked path is the one already tested on the compound page. Nothing at the start, $100.00 at the end of each month, 5 percent compounded monthly, ten years, ends at $15,528.23. Ask this page for that ending balance, at the same rate and the same ten years, and the deposit comes back as $100.00. Five percent is not a savings-account quote and not a stock-market average. It is the rate that makes the two pages agree. Investor.gov's compound calculator takes an initial amount, a monthly contribution, a rate, and a compounding frequency. This page always compounds monthly, for the same reason the compound page does: a monthly deposit and a monthly compound period do not need an extra rule about where inside the year the money arrives.
What the goal is not
The goal is a nominal balance, in the dollars you typed. It is not a purchasing-power target. If prices rise, the same balance buys less. The inflation page divides a sum by (1 + rate) to the years. You can take a future cost from that page and paste it here as the goal, but only if you want the deposit to aim at that inflated number. This page will not inflate the goal on its own, because it would have to pick a rate. The Federal Reserve describes inflation as a general increase in the overall price level, measured by price indexes that can disagree. A single percent in a form is not one of those indexes.
Taxes are not taken out of the interest. A taxable account can owe tax on interest each year, which lowers the rate you actually keep. Type a net rate if that is the question. A retirement account has a different tax timing and a legal contribution limit. The retirement page caps elective deferrals at the 2026 IRS limit and then compounds. This goal page will happily ask for a deposit no plan would accept. That is a feature of a general goal and a warning not to treat the deposit as a 401(k) instruction.
Rounding and the last cent
The unrounded deposit that lands exactly on an unrounded future value can be a fraction of a cent. The page rounds the deposit to the cent and then grows that rounded deposit. The ending balance can therefore miss the goal by a cent. On the checked example, the rounded deposit of $100.00 reaches $15,528.23, which is the goal that was typed. If you type a goal that is not itself a rounded compound result, look at the ending balance, not only the deposit. The ending balance is the one the forward formula produces.
Deposits are level. The page does not step them up with inflation, with a raise, or with a bonus. A bonus you plan to add once is part of the starting amount only if you already have it. A raise belongs in a different model. Level deposits are the model Investor.gov's monthly contribution field also uses: one amount, every month. If your saving will be irregular, this single number is an average you chose, and the balance will not match a year when you skip a month.
How to read a large deposit
A short horizon and a high goal produce a large deposit. That is arithmetic, not a judgment that the goal is wrong or that you should lengthen the horizon. Lengthening the horizon lowers the deposit and increases the role of the assumed rate. Raising the rate lowers the deposit and increases the chance the assumption is the thing that fails. The page will not mark a rate as conservative or aggressive. Those words would be a market opinion. Giuseppe Ilarda, the site owner, writes and checks the algebra. The disclaimer says the result is not financial advice. The editorial policy covers the loan formulas. This future-value inverse is a separate function, tested against the forward compound result so the two pages cannot drift apart by a silent change in rounding.