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?
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.
You've hit on something real here. I ran into this exact mess when a mate was looking at refinancing, and we started poking around different calculators just for curiosity. The spread you're seeing - from £1,389 to £1,512 - that's not rounding. That's probably a mix of two things: the mortgage term isn't standardized across all three, and they might be handling additional costs differently.
If one calculator is assuming 25 years and another is using 20 years, you'll absolutely see that kind of gap. A 5-year difference on a £285k loan at 4.5% will shift your payment by maybe £80-100 per month. It sounds like the other replies already mentioned this, which makes sense because it's the biggest culprit. But beyond the term itself, check whether these calculators are including things like arrangement fees upfront, insurance products, or whether some are asking about your deposit amount and calculating based on the actual borrowed sum. A couple of them might also be showing you interest-only vs. repayment, though that's usually pretty obvious if you look at the output.
My advice: pick one calculator - ideally from your actual lender if you're that far along - and stick with it. Make sure you know what term length it's assuming (usually they default to 25 years in the UK) and whether it's just showing you principal and interest or bundling other stuff in. Once you've got those details locked down, the number should be consistent. If your lender gives you a formal illustration, that's your real baseline anyway, not these online quick-checks.
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.
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