Interest Calculator

How this result is built

See the interest a starting balance and ongoing contributions earn at a compound rate.

The ending balance is the initial investment plus every annual and monthly contribution, grown at the compound frequency you pick. Beginning-of-period contributions earn interest in the period they are deposited. End-of-period contributions wait until the next one.

What the opening example is for

Tax, when you enter it, is removed from each period’s interest before that interest joins the balance. Inflation does not change the account. It only restates the ending balance in today’s buying power. Leave inflation at zero when you want the nominal balance alone.

The walkthrough below uses the numbers already sitting in Interest Calculator. Press Calculate and the headline should match this page. Then put your own figures in. The math does not run until you press the button, and Clear hides the result.

What each field is doing

The “Initial investment” line on Interest Calculator starts at $20,000.00. The other fields stay put while you change this one. That is the clean way to see what this line is doing inside the result.

Interest Calculator treats “Annual contribution” as its own entry, and the form opens with $5,000.00. If the label is a rate, type the annual figure the contract uses. If it is money, type the amount that will actually move, not a rounded hope.

Before you replace it, “Monthly contribution” reads $0.00 on Interest Calculator. Leave that figure only when it matches the statement, the quote, or the plan you are testing. Press Calculate after the edit so the chart and the schedule move with it.

Look at “Contribute at the” first if you are checking Interest Calculator: the filled value is Beginning of each compounding period. A round example is useful for learning the page. It is a poor substitute for the number on your own paperwork, so replace it and run the form again.

The “Interest rate” line on Interest Calculator starts at 5%. The other fields stay put while you change this one. That is the clean way to see what this line is doing inside the result.

The remaining fields

Interest Calculator treats “Compound” as its own entry, and the form opens with Annually. If the label is a rate, type the annual figure the contract uses. If it is money, type the amount that will actually move, not a rounded hope.

Before you replace it, “Years” reads 5 on Interest Calculator. Leave that figure only when it matches the statement, the quote, or the plan you are testing. Press Calculate after the edit so the chart and the schedule move with it.

Look at “Months” first if you are checking Interest Calculator: the filled value is 0. A round example is useful for learning the page. It is a poor substitute for the number on your own paperwork, so replace it and run the form again.

The “Tax rate on interest” line on Interest Calculator starts at 0%. The other fields stay put while you change this one. That is the clean way to see what this line is doing inside the result.

Interest Calculator treats “Inflation rate” as its own entry, and the form opens with 3%. If the label is a rate, type the annual figure the contract uses. If it is money, type the amount that will actually move, not a rounded hope.

The opening result, line by line

Press Calculate on the opening form and Ending balance is $54,535.20. That is the number the rest of this section is talking about.

Total principal is $45,000.00 when you run the opening figures on Interest Calculator. That line is part of the same calculation as the headline. It is not a second balance you can spend on top of the result. Change one input and press Calculate if you want to see whether this line moves.

On this opening pass, total contributions is $25,000.00. That line is part of the same calculation as the headline. It is not a second balance you can spend on top of the result. Change one input and press Calculate if you want to see whether this line moves.

Interest Calculator reports that total interest is $9,535.20. That line is part of the same calculation as the headline. It is not a second balance you can spend on top of the result. Change one input and press Calculate if you want to see whether this line moves.

Interest of initial investment is $5,525.63 when you run the opening figures on Interest Calculator. That line is part of the same calculation as the headline. It is not a second balance you can spend on top of the result. Change one input and press Calculate if you want to see whether this line moves.

How those lines were produced

On this opening pass, interest of the contributions is $4,009.57. That line is part of the same calculation as the headline. It is not a second balance you can spend on top of the result. Change one input and press Calculate if you want to see whether this line moves.

Interest Calculator reports that buying power after inflation is $47,042.54. That line is part of the same calculation as the headline. It is not a second balance you can spend on top of the result. Change one input and press Calculate if you want to see whether this line moves.

Contributions are added at the beginning of each compounding period, so they earn interest in that period. Buying power divides the ending balance by inflation over the same term. It is not a second investment return.

The formula line on the page is Balance grows by the compound rate you chose. Tax, when entered, is taken out of each period’s interest.. It is there so you can see the relationship the buttons are using. A schedule, when the page draws one, is that same series written row by row.

A second pass changes only “Initial investment”, from $20,000.00 to $22,000.00. Ending balance moves from $54,535.20 to $57,087.76. Nothing else on the form was edited, so the gap between those two results is the effect of that one field on Interest Calculator. If the gap is tiny, this page is not very sensitive to that entry. If the gap is large, get that entry from a document before you plan around the result.

What this page will not claim

Interest Calculator compounds the rate you type. Markets do not pay a flat rate, and a fund fee comes out of the return before you should type it. The page is the equation, not a forecast and not a recommendation to buy or sell.

Interest Calculator stops at its own formula. When the next question is different, open Compound Interest Calculator (Interest that starts earning interest of its own.); Savings Calculator (What a monthly deposit becomes.); CD Calculator (APY in, value at maturity out.). Do not add those results to this one as if they were the same kind of dollar.

Check the result before you use it

Start on Interest Calculator by reading the labels, not the headline. The opening labels are Initial investment ($20,000.00), Annual contribution ($5,000.00), Monthly contribution ($0.00), Contribute at the (Beginning of each compounding period), Interest rate (5%), Compound (Annually), Years (5), Months (0), Tax rate on interest (0%), Inflation rate (3%). If a label is not the fact you have, the result cannot turn into that fact later. The illustration on the form lands on $54,535.20. When you press Calculate and see that same figure, the button and this explanation are using one formula.

Change one field at a time. Write down the headline, edit a single line, and press Calculate again. A large jump means that line carries the result. A line that does not move is a line the formula ignores, which is a reason to confirm you opened the right page. Interest Calculator is built for one job: A direct answer to how much interest you earn. The description on the page says the same thing in a sentence: See the interest a starting balance and ongoing contributions earn at a compound rate.

When the paperwork disagrees

Then hold the headline next to a source outside the site. That source might be a loan estimate, an account statement, a pay stub, or the public limit named in the methodology. A few cents of difference on a long schedule is ordinary rounding. A different rate, a fee you never typed, or a tax the form does not ask for is not rounding. Correct the input. Do not average the page with the paperwork.

When the outside source still disagrees, Interest Calculator has reached the edge of what it will claim. It will not phone a lender, pull a county record, or apply a credit you left blank. Send the calculator name, every input, and the result you expected through the contact page if you think the formula itself is wrong. Until that is settled, read $54,535.20 as the output of the fields above and nothing else.

FAQs