Savings with Monthly Deposits Calculator
Saving is rarely a one-time event; most people start with some money and then add to it month after month. This calculator projects what that habit grows into. It combines two engines of growth: the compound interest earned on your starting balance, and the future value of every monthly deposit you make along the way. Each deposit lands in a different month and so earns interest for a different length of time, which is exactly what the annuity formula captures. The earlier a dollar goes in, the longer it compounds and the more it becomes by the end. Enter your starting balance, the amount you plan to deposit each month, the annual interest rate and the number of years, and the tool returns the projected balance, splitting out how much you contributed versus how much the account earned. Seeing those two numbers side by side often surprises people: over a long horizon the interest can rival or exceed the deposits themselves. This is the mathematical case for starting to save steadily and early. Every figure is computed deterministically from the standard future-value formulas, with a worked example below that reconciles exactly to the calculator so you can check each step.
Future value of a starting balance plus monthly deposits compounds both streams: FV = P(1 + r)^n + PMT[((1 + r)^n - 1) / r]. Starting with 1,000, adding 200 a month at 5% for 10 years grows to $32,703.47.
Savings with deposits formula
FV = P(1 + r)^n + PMT x ( ((1 + r)^n - 1) / r )
P = starting balance
PMT = monthly deposit (end of month)
r = monthly rate (annual / 12, as a decimal)
n = number of months (years x 12)
The first term compounds the starting balance. The second term is the future value of an ordinary annuity, the sum of every deposit grown to the end of the period.
Worked example
Start with 1,000, deposit 200 each month, at 5% per year for 10 years.
- Monthly rate r = 0.05 / 12 = 0.0041667; months n = 120.
- (1 + r)^n = 1.0041667^120 = 1.647009.
- Starting balance grows to 1,000 x 1.647009 = 1,647.01.
- Deposits grow to 200 x ((1.647009 - 1) / 0.0041667) = 200 x 155.282 = 31,056.46.
- Future value = 1,647.01 + 31,056.46 = 32,703.47.
Total deposited is 1,000 + 200 x 120 = 25,000, so interest earned is 32,703.47 - 25,000 = 7,703.47. These are the calculator's default inputs, so the result above matches the widget exactly.
Savings with monthly deposits calculator: frequently asked questions
How does this calculator combine a starting balance and monthly deposits?
It runs two future-value calculations and adds them. The starting balance grows by compound interest over the full period. The monthly deposits form an ordinary annuity, each deposit compounding from the month it is made. Adding the grown starting balance to the future value of the deposit stream gives the total.
What is compound interest?
Compound interest is interest earned on both your original money and on the interest already credited. Each period the balance is multiplied by one plus the periodic rate, so growth accelerates over time. The more often interest compounds, the faster a balance grows for the same nominal rate.
Does the deposit timing matter?
Yes. This calculator assumes deposits are made at the end of each month, the ordinary annuity convention. Depositing at the start of each month, an annuity due, gives a slightly higher result because each deposit earns one extra month of interest. The difference is small but real.
How big a difference do regular deposits make?
Often a very large one. Over long periods the steady stream of deposits usually contributes more to the final balance than the starting amount, because every deposit gets time to compound. This is why starting to save regularly early matters more than the size of any single deposit.
What is the savings with deposits formula?
Future value equals P times (1 + r) to the power n, plus PMT times (((1 + r) to the power n minus 1) divided by r). P is the starting balance, PMT is the monthly deposit, r is the monthly rate, and n is the number of months. The first term grows the principal and the second grows the deposit stream.
Official sources
- Saving, compound interest and the future value of regular contributions: US Securities and Exchange Commission, Investor.gov. As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.