15 vs 30 Year Mortgage Calculator
A shorter term costs more every month and far less over the loan. This puts both figures next to each other so the trade is a number rather than an argument.
Interest saved
$229,863.03by the shorter term
- 30-year payment
- $1,896.20
- 15-year payment
- $2,515.39
- 30-year interest
- $382,633.47
- 15-year interest
- $152,770.44
- Extra each month
- $619.19
- Interest saved
- $229,863.03
- The shorter term costs $619.19 more each month and saves $229,863.03 in interest — roughly 2.06 saved for every extra pound or dollar paid.
- Shorter terms usually carry a slightly lower rate as well, so set the two rates separately rather than assuming they match.
- A shorter term is a commitment, not a preference. Taking the longer term and overpaying voluntarily gives most of the saving with room to stop in a bad month.
About the 15 vs 30 year comparison
On $300,000 the fifteen-year term costs $619 a month more and saves $229,863 in interest. Put another way, every extra pound paid in comes back about twice over — a return most investments would struggle to guarantee.
That framing understates the case slightly, because a shorter term usually carries a lower rate as well. Lenders price fifteen-year money below thirty-year money, often by half a point or more, so the comparison here lets you set the two rates separately rather than assuming they match.
The argument against is not financial, it is structural. A fifteen-year payment is a commitment: you owe it every month whatever else happens. A thirty-year mortgage with voluntary overpayments reaches nearly the same place with a floor you can drop back to in a bad year. The thirty-year term costs more if you keep overpaying and rescues you if you cannot.
Which is right depends less on the arithmetic than on how stable the income is and whether the extra $619 would otherwise be doing something more useful — clearing higher-rate debt, or going into a retirement account with an employer match, both of which beat a mortgage rate comfortably.
What it works out
- Both monthly payments, at their own rates
- Total interest for each term
- What the shorter term costs each month
- Any two terms, not just 15 and 30
The formula
Both terms use M = P · r(1+r)ⁿ ÷ ((1+r)ⁿ − 1), with their own r and n
Nothing new here — it is the standard payment formula run twice, once for each term, then subtracted.
$300,000 at 6.5% over thirty years is $1,896.20 a month and $382,633 of interest. The same amount at 5.9% over fifteen years is $2,515.39 a month and $152,770 of interest. The shorter term costs $619.19 more monthly and saves $229,863.
The saving is that large for two compounding reasons. Half the months means far less time for interest to accrue, and the lower rate applies to every one of those months. Neither effect is dramatic on its own; together they halve the cost of the house.
The rate difference is worth setting honestly. Fifteen-year money is genuinely cheaper — lenders take less duration risk — so using the same rate for both understates the shorter term's advantage.
- P
- The amount borrowed. The same for both, since it is the same house.
- r
- The monthly rate for that term. Shorter terms usually price lower.
- n
- Months in the term: 180 for fifteen years, 360 for thirty.
A worked example
$300,000 over thirty years at 6.5%, against fifteen years at 5.9%.
That works out to $229,863.03 by the shorter term.
- 30-year payment
- $1,896.20
- 15-year payment
- $2,515.39
- 30-year interest
- $382,633.47
- 15-year interest
- $152,770.44
- Extra each month
- $619.19
- Interest saved
- $229,863.03
Questions
How much more is a 15 year mortgage each month?
On $300,000 at 6.5% against 5.9%, about $619 a month more — $2,515 against $1,896. The proportion holds at other loan sizes: roughly a third more each month.
How much does a 15 year term save?
$229,863 in interest on a $300,000 loan at those rates. That is most of the price of the house again, and it comes from halving the number of months rather than from any trick.
Is a 15 year mortgage always better?
Financially, nearly always. Practically, not always. The higher payment is compulsory every month for fifteen years, and a thirty-year loan you overpay voluntarily gets most of the same benefit while leaving you a floor to fall back to if income drops.
Why is the rate lower on a shorter term?
The lender is exposed for half as long, so it takes less duration and inflation risk and prices accordingly. Half a point is a typical gap, and it is worth getting both quotes rather than assuming.
Should I take 30 years and overpay instead?
It is a genuinely good option. You reach a similar place with the option to stop in a difficult year, at the cost of a slightly higher rate and the risk that you do not actually keep overpaying. The discipline is the real variable.
What else could that extra $619 do?
Clearing higher-rate debt beats it outright — no mortgage rate approaches a credit card. So does an employer pension or 401(k) match, which is an immediate return no loan can match. Below those two, overpaying is strong.
Does a 20 year term make sense?
Often, and it is under-used. It captures much of the interest saving without the full jump in payment. Enter twenty as the shorter term here to see where it lands.
Can I switch from 30 to 15 later?
Only by refinancing, which has its own costs and depends on rates at the time. Overpaying a thirty-year loan achieves the same shortening without needing a new loan at all.