I've been trying to figure out my monthly payments on a £285k house with a 4.5% rate. I used three different online calculators and got three different answers - one said £1,447, another £1,389, and a third £1,512. Is it just rounding errors or am I missing something about how interest compounds?
This is frustrating but there's actually a few things going on here. The main culprit is usually the loan term - if you're not specifying exactly 25 years vs 30 years or whatever, you'll get wildly different monthly figures. Even a difference of 5 years can swing your payment by over £100. Also some calculators let you set start dates and they might calculate interest differently depending on when payments begin. Then theres the stuff about whether it's calculating APR vs the actual interest rate, some have fees baked in, and honestly some online calculators are just poorly designed and use outdated formulas. I've helped friends with their mortgages and seen the exact same thing happen.
The compounding question you're asking about is actually simpler than this - most mortgages use monthly compounding so the interest divides evenly month to month. That's pretty standard. What matters more is making sure your calculator knows: the exact loan amount, your interest rate, the number of months you're paying over, and whether there are any other fees included. With £285k at 4.5% over 25 years youre looking at around £1,447 roughly, but I'd double-check that 25 year assumption. If one calculator used 30 years that would explain the lower £1,389 figure pretty easily. Just plug in the exact same numbers to each one and see if they match - if they still don't, thats when you know somethings off with one of the calculators. Your bank or mortgage broker will give you the real answer anyway, they have to be accurate.
The other person nailed it - term length is gonna be your biggest swing here. But also watch out for whether the calculator's including fees, insurance, taxes, or just the principal and interest. Some of them default to different assumptions too, like whether you're putting down a deposit or going full 285k, and if there's an arrangement fee baked in. Best bet is pick one reputable calculator (your bank's usually solid) and stick with it while you're shopping around, then get an actual mortgage advisor to give you the real figure since they'll factor in everything specific to your situation.
The term length is definitely your biggest issue - those calculators probably aren't all assuming the same number of years, and even a few years difference will swing your monthly payment by £50-100 easily. as some answers noted, you also gotta check if they're including extras like fees, insurance, or property tax, which some calculators bundle in and others don't. The compounding method shouldn't really cause that much variance tbh, but make sure you're plugging in exactly the same inputs (loan amount, interest rate, and most importantly the full term in years) and then the numbers should line up way better. Honestly I'd reccommend just sticking with one calculator once you've got the inputs right rather than switching between them - it'll drive you mad lol.
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