--- title: "How to Calculate Loan Amortization (Step by Step)" site: ProCalc.ai type: Blog Post category: how-to domain: Finance url: https://procalc.ai/blog/how-to-calculate-loan-amortization-step-by-step markdown_url: https://procalc.ai/blog/how-to-calculate-loan-amortization-step-by-step.md date_published: 2026-03-14 date_modified: 2026-03-18 read_time: 6 min tags: loan-amortization, personal-finance, mortgage --- # How to Calculate Loan Amortization (Step by Step) **Site:** [ProCalc.ai](https://procalc.ai) — Free Professional Calculators **Category:** how-to **Published:** 2026-03-14 **Read time:** 6 min **URL:** https://procalc.ai/blog/how-to-calculate-loan-amortization-step-by-step > *This file is served for AI systems and search crawlers. Human page: https://procalc.ai/blog/how-to-calculate-loan-amortization-step-by-step* ## Overview A practical, step-by-step way to calculate loan amortization, build a schedule, and see how extra payments change your payoff date and total interest. ## Article I was staring at my own loan spreadsheet and it still felt like magic I was sitting at my kitchen table with a mug of coffee, a half-finished spreadsheet, and a loan statement that kept insisting I owed basically the same amount month after month. And I’m thinking, “Wait… I’m paying every month. Why does the balance barely move?” I’d built models for work, I’d managed my own money for years, and still, amortization felt like one of those words people nod at like they get it (I nodded too). It took me a while to realize the math wasn’t hard — it’s just annoyingly easy to do slightly wrong. So if you’re trying to compare two loans, figure out how much interest you’ll pay, or see what happens if you throw an extra 200 at the principal each month, this is the step-by-step that actually matches how you’d use it in real life. And yes, the balance drops… just not right away. Amortization, in plain language (not the textbook version) The thing is, a loan payment is usually two things smashed together: interest and principal. Interest is the “rent” you pay to borrow the money, and principal is the part that actually reduces what you owe. Early in the loan, the interest portion is big because your balance is big. Later, the interest portion shrinks because the balance shrinks. That’s the whole trick. A lot of people think “fixed payment” means “fixed interest.” It doesn’t. The payment stays the same, but the mix changes every month. And a quick definition that clears up a ton: an amortization schedule is just a month-by-month table that shows payment, interest, principal, and remaining balance. That’s it — one schedule, a bunch of rows. If you want to jump straight to tools, I built these because I got tired of re-checking my own spreadsheets: Loan amortization calculator (schedule + totals) Loan payment calculator if you’re starting from scratch Mortgage calculator Extra payment calculator for “what if I pay extra?” scenarios Refinance calculator for comparing a new loan against your current one Simple interest calculator for the rare cases where amortization isn’t even the right model Here’s an embedded version if you just want to plug numbers in and see the schedule. The step-by-step: how to calculate loan amortization by hand (without losing your mind) I’m going to use a real-ish scenario: a 300,000 loan, 30 years, 6.5% annual interest, paid monthly. That’s a pretty normal “mortgage math” example, but the same steps work for car loans, personal loans, whatever — as long as it’s an amortizing loan with fixed payments. So what do you actually calculate each month? Figure out the monthly payment (fixed). Compute that month’s interest (balance × monthly rate). Principal paid = payment − interest. New balance = old balance − principal paid. Repeat for the next month (and watch the interest portion slowly shrink). 💡 THE FORMULA Monthly Payment = P × [ r(1+r)^n ] / [ (1+r)^n − 1 ] P = loan principal (starting balance) r = monthly interest rate (annual rate ÷ 12) n = total number of payments (months) Now let’s actually run the numbers, because that’s where it clicks. Inputs (rounded a bit): Principal P = 300,000 Annual rate = 6.5% so monthly r = 0.065 / 12 = about 0.0054167 Term = 30 years so n = 360 payments Step 1: Monthly payment Using the formula above, the payment comes out around 1,896 per month (give or take a couple bucks depending on rounding). If you’ve ever looked at a mortgage quote and thought “that seems low for a 300,000 loan,” remember: that’s principal + interest only, not taxes/insurance/HOA (those are separate buckets). Step 2: Month 1 interest Interest = balance × r = 300,000 × 0.0054167 ≈ 1,625 Step 3: Month 1 principal Principal = payment − interest = 1,896 − 1,625 ≈ 271 Step 4: New balance New balance ≈ 300,000 − 271 = 299,729 So yeah… you pay almost 1,900 and only about 270 actually knocks down the balance. That’s the part that makes people mad (and honestly, I get it). But it’s not a scam — it’s just how amortization works when the balance is big and the rate is non-trivial. Now do month 2 the same way, but using the new balance. The interest drops a tiny bit, principal rises a tiny bit, and the schedule slowly tilts in your favor. Here’s what the first few months look like with rounded numbers (don’t obsess over the pennies; your lender’s schedule will be exact to the cent, but the pattern is what you’re after). Month Payment Interest Principal Remaining Balance 1 1,896 1,625 271 299,729 2 1,896 1,623 273 299,456 3 1,896 1,622 274 299,182 12 1,896 about 1,607 about 289 about 296,700 And here’s the part most people miss: amortization is a loop. Once you have the payment, the rest is just repeating steps 2–4 for 360 rows. That’s why calculators are handy — not because it’s “advanced,” but because nobody wants to copy formulas down a spreadsheet 360 times and then wonder if they referenced the wrong cell. But if you’re building your own model (or checking a lender), the monthly interest line is the tell. If the interest isn’t exactly prior balance × monthly rate, something’s off. So why does everyone get this wrong? Because they try to compute principal first. Interest comes first. Always. The part that saves you real money: extra payments (and how to model them) I’m going to be a little opinionated here: if you’re going to “pay extra,” you should be crystal clear about what your lender does with it. Some places apply it to next month’s payment (which feels nice but doesn’t always reduce interest much). What you typically want is extra principal reduction — meaning it immediately lowers the balance, which lowers future interest. Here’s the practical way to model it in your amortization schedule: Keep your normal payment the same. Add an “extra principal” column (like 200 per month, or maybe 5,000 once a year). Each month: New balance = Old balance − (scheduled principal + extra principal). And you’ll see something kind of satisfying: the schedule ends early. Like, not “a little early.” Sometimes years early. That’s a lot of interest you don’t pay! If you want to run the “what if I pay an extra 200 monthly?” scenario fast, use this extra payment calculator . If you want the full schedule view, the amortization calculator is the one. One more thing (because I’ve personally tripped on this): if you’re comparing “extra payment” vs “invest the difference,” that’s not an amortization question. That’s a separate return-and-risk conversation. Amortization will tell you the guaranteed savings in interest, which is still super useful, but it won’t tell you what the market does next year. Common gotchas I’ve seen (including in my own sheets) But the spreadsheet only works if your assumptions match reality. Here are the mistakes that keep showing up, over and over: Using annual rate as monthly rate. If you use 0.065 instead of 0.065/12, your schedule will look like a disaster. Mixing compounding conventions. Most consumer loans are quoted as an annual rate but applied monthly. Keep it consistent. Forgetting taxes and insurance aren’t part of amortization. Your “total payment” might be 2,450, but amortization is only the loan part. Rounding too early. Round for display, not for the actual math, or your ending balance will be weirdly not-zero. Refi comparisons without fees. If you’re using a refinance calculator , don’t pretend closing costs don’t exist (they do, and they matter). So yeah, if your schedule ends with a balance of 43.12 after 360 payments, it’s probably just rounding. But if it’s off by 4,300, something’s broken. FAQ Why is my loan balance barely going down in the first year? Because interest is calculated on the current balance, and early on that balance is huge. Your payment is fixed, so a bigger interest chunk leaves a smaller principal chunk. Over time the balance drops, interest drops, and principal ramps up. Is amortization the same thing as simple interest? Nope. Simple interest usually means interest is computed in a straightforward way without the same month-by-month principal/interest split that a fixed-payment amortizing loan has. If you’re not sure which you’re dealing with, run both: simple interest vs amortization . How do I know if refinancing makes sense? Compare your current remaining balance, rate, and remaining term to the new offer. Include fees (origination, appraisal, title, whatever applies). Look for a breakeven month: when cumulative savings exceed total refi costs. If you want a quick run, use the refinance calculator and sanity-check the new payment with the payment calculator . If you only take one thing from all this: amortization is just repeating the same three lines — interest, principal, new balance — and the “mystery” goes away once you see month 1 and month 2 side by side. And if you don’t want to build the sheet, that’s fine too. That’s literally why I built ProCalc.ai. --- ## Reference - **Blog post:** https://procalc.ai/blog/how-to-calculate-loan-amortization-step-by-step - **This markdown file:** https://procalc.ai/blog/how-to-calculate-loan-amortization-step-by-step.md ### AI & Developer Resources - **LLM index:** https://procalc.ai/llms.txt - **LLM index (full):** https://procalc.ai/llms-full.txt - **MCP server:** https://procalc.ai/api/mcp - **Developer docs:** https://procalc.ai/developers ### How to Cite > ProCalc.ai. "How to Calculate Loan Amortization (Step by Step)." ProCalc.ai, 2026-03-14. https://procalc.ai/blog/how-to-calculate-loan-amortization-step-by-step ### License Content © ProCalc.ai. Free to reference and cite. Do not republish in full without attribution.