The formula
College saving is three formulas: the future value of what you already have, the amount still to save, and the monthly payment that covers it. The future value of your current savings is the current savings times one plus the return, raised to the years. The amount still to save is the target cost minus that future value. The monthly payment (end of each month, monthly compounding) is the amount still to save times the monthly rate, divided by one plus the monthly rate raised to the months, minus one — where the monthly rate is the annual return divided by 12 and the months are the years times 12. If the return is 0%, the monthly payment is simply the amount still to save divided by the months. The college savings calculator runs the same seven steps for you.
Step 1 — Pick the target cost
Start with the total cost of the education. Our worked example uses a $120,000.00 target: a four-year degree at an in-state public university commonly costs past $100,000 all-in (tuition, fees, room and board over four years), and private colleges cost far more. Pick the number for the school you have in mind: college costs rise roughly 3% a year, faster than general inflation, so a $120,000.00 target today is bigger by enrolment. Write the target down first: $120,000.00.
Step 2 — Count the years
Count how many years until the money is needed. Our worked example uses 10 years: a child who is eight today starts college in ten years. Ten years is 120 months, setting both how long your savings grow and how many payments you make. A shorter horizon means fewer payments and less growth, which is why the monthly amount climbs so fast as enrolment gets closer.
Step 3 — Work out what your savings will grow to
Multiply your current savings by one plus the return, raised to the years. On the worked example that is $10,000.00 × 1.06^10 = $17,908.48. This is the start-early rule in one number: the $10,000.00 already saved grows to $17,908.48 on its own, without another dollar added. A 6% long-run return is a common planning assumption for a balanced investment mix, but it is an assumption, not a promise. If you have nothing saved yet, this step is $0.00 and the whole target is still to save.
Step 4 — Subtract for what is still to save
Subtract the future value of your savings from the target cost. On the worked example that is $120,000.00 − $17,908.48 = $102,091.52. This is what the monthly payments must cover. If your savings already cover the target, the answer here is zero or less and the monthly amount is $0.00: with $150,000.00 already saved it grows to $268,627.15, already more than the $120,000.00 target, so nothing more to save.
Step 5 — Work out the monthly amount
Spread the amount still to save over the months at the monthly rate. The monthly rate is the annual return divided by 12 — 6% ÷ 12 = 0.5% a month — and the months are the years times 12, so 120 months. On the worked example the monthly amount is $102,091.52 × 0.005 ÷ ((1.005)^120 − 1) = $622.97. A higher assumed return lowers the monthly amount, because growth does more of the work — at 8% the same target takes $537.92 a month — but it is only a guess about the future, so a conservative assumption is the safer plan.
Step 6 — Check the total you will pay in and the interest
Multiply the monthly amount by the months for the total you will pay in, then subtract that and your current savings from the target for the interest earned. On the worked example the total paid is $622.97 × 120 = $74,756.40, and the interest earned is $120,000.00 − $10,000.00 − $74,756.40 = $35,243.60. The check is the three numbers adding back to the target: $10,000.00 + $74,756.40 + $35,243.60 = $120,000.00. If they do not add up, one of the five numbers is wrong, and that is the number to re-check.
Step 7 — Check the result against the college savings calculator
Run the same numbers through the college savings calculator and check that it gives the same $17,908.48, $102,091.52, $622.97, $74,756.40 and $35,243.60 — if it does not, one of the five numbers is wrong, and that is the number to re-check. Then try the edges: the smallest whole-dollar target for the monthly amount to reach $1,000.00 is $181,788; the smallest whole-dollar current savings for the monthly amount to fall to $500.00 or less is $21,253; and $67,007 already saved makes the monthly amount $0.00. A 529 plan lets savings grow tax-free when spent on education — the 529 plan calculator covers the tax side.
Three worked examples
Five years left. With 5 years instead of 10, the $10,000.00 grows to only $13,382.26, leaving $106,617.74 to save in 60 months — $1,528.13 a month, nearly two and a half times the 10-year figure. Waiting is expensive: the same target in half the time costs two and a half times as much a month.
Eighteen years from birth. The $10,000.00 grows to $28,543.39 over 18 years, leaving $91,456.61 to save over 216 months — $236.11 a month, less than half the 10-year figure. The extra eight years do two jobs at once: they grow the savings you already have and they spread the rest over 96 more payments.
A higher return. Assume 8% instead of 6% and the $10,000.00 grows to $21,589.25, leaving $98,410.75 to save — $537.92 a month. The return rule cuts both ways: a higher assumed return lowers the monthly amount on paper, but if the return does not arrive, the fund falls short by exactly the missing growth.
The start-early rule
Starting early matters more than saving big, because compounding does the heavy lifting over a long horizon. The $10,000.00 already saved grows to $17,908.48 in 10 years and to $28,543.39 in 18 years — the same $10,000.00, nearly three times the growth, just for starting eight years earlier. That is why the monthly amount falls from $622.97 to $236.11 over 18 years instead of 10: the savings you already have do more, and the rest spreads thinner. The best month to start saving for college is the month the child is born; the second best is this one.
What the monthly amount does not tell you
The monthly amount is a plan, not a promise: the 6% return is an assumption, and a bad decade can leave the fund well short. It also hides the target moving — college costs rise roughly 3% a year, so a $120,000.00 target in 10 years may really be $160,000.00. The figures here are before taxes and fees, and they assume you save the same amount every month, which a real budget rarely allows. The compound interest calculator shows the growth. Use the monthly amount to set the habit — then re-check the target and the return every year.