Methodology & Sources

How our calculators work

Every tool on this site runs entirely in your web browser using JavaScript. Nothing you type into a calculator is uploaded to a server, stored or sold. Our finance calculators use the same standard formulas that lenders and financial professionals use, and every tool page explains its formula in plain English. Where a result depends on published data — tax bands, mortgage rates, insurance averages — we use the official source and name it below.

Loan and mortgage formulas

Our loan, mortgage, auto loan, personal loan and EMI calculators use the standard amortization formula for a fixed-rate, fully amortizing loan:

M = P × r(1+r)ⁿ ÷ ((1+r)ⁿ − 1)

where M is the monthly payment, P is the loan amount, r is the monthly interest rate (annual rate ÷ 12) and n is the number of monthly payments. For example, a $300,000 loan at 6.5% over 30 years (360 payments) gives a monthly principal-and-interest payment of $1,896.20. The amortization schedule calculator applies the same formula month by month: each payment covers that month's interest first, and the rest reduces the balance — which is why early payments are mostly interest and later payments are mostly principal.

Our mortgage calculator adds property tax, homeowners insurance, PMI and HOA dues on top of principal and interest. PMI is estimated at 0.5% of the loan amount per year when the down payment is under 20%; your lender's actual PMI rate may be higher or lower. The refinance calculator compares your current loan with a new one and divides the closing costs by the monthly saving to find the break-even point in months.

Compound interest and savings

Savings growth uses the standard compound interest formula, A = P(1 + r/n)^nt, where A is the final amount, P is the starting amount, r is the annual rate, n is the number of compounding periods per year and t is the number of years. Regular monthly contributions are added to the balance each month and then earn interest in later months. The savings goal calculator works the same formula backwards to find the monthly amount needed to reach a target by a chosen date.

UK tax, National Insurance and Stamp Duty

Our UK salary calculator uses the 2026/27 PAYE rules (tax year 6 April 2026 to 5 April 2027) and lets you choose your region: England, Wales and Northern Ireland, or Scotland. For England, Wales and Northern Ireland: a standard Personal Allowance of £12,570, Income Tax at 20% on income from £12,571 to £50,270, 40% from £50,271 to £125,140, and 45% above that. For Scotland, the calculator applies the six Scottish bands: starter 19% (£12,571–£16,537), basic 20% (£16,538–£29,526), intermediate 21% (£29,527–£43,662), higher 42% (£43,663–£75,000), advanced 45% (£75,001–£125,140) and top 48% (over £125,140). The Personal Allowance (£12,570, falling by £1 for every £2 of income above £100,000 and reaching zero at £125,140) and Employee Class 1 National Insurance (8% on earnings between £12,570 and £50,270 a year, 2% above that) are the same UK-wide, so only the income tax bands change when you switch regions. Sources: GOV.UK — Income Tax rates and Personal Allowances, GOV.UK — National Insurance and mygov.scot — Scottish income tax, current rates.

Our UK mortgage calculator estimates Stamp Duty Land Tax (SDLT) on the slice system for England and Northern Ireland: 0% on the first £125,000, 2% on the portion from £125,001 to £250,000, 5% from £250,001 to £925,000, 10% from £925,001 to £1.5 million, and 12% above that. First-time buyers pay no SDLT on the first £300,000 of a home costing £500,000 or less, and 5% on the portion from £300,001 to £500,000; above £500,000 the relief is lost entirely. Source: GOV.UK — Stamp Duty Land Tax.

US tax figures

Our US tax reference tables use the official IRS rate schedules and standard deductions: Revenue Procedure 2025-32 for tax year 2026 and Revenue Procedure 2026-35 for tax year 2027. The seven federal rates (10%, 12%, 22%, 24%, 32%, 35%, 37%) apply to slices of taxable income — income after the standard deduction or itemized deductions. Social Security tax is 6.2% on wages up to the SSA wage base ($184,500 in 2026; $192,300 in 2027), and Medicare tax is 1.45% on all wages. Sources: IRS Revenue Procedure 2025-32 (PDF), IRS Revenue Procedure 2026-35 (PDF) and SSA — Contribution and Benefit Base.

Our US salary calculator uses the same federal model: choose tax year 2026 or 2027 and filing status (single or married filing jointly), and it applies the real federal brackets and standard deduction, Social Security tax up to the wage base, and Medicare tax (including the 0.9% Additional Medicare Tax above $200,000 single / $250,000 married filing jointly). It also models state income tax for five states — California, New York, Illinois, Texas and Florida — using 2026 single-filer rules (Texas and Florida have no state income tax). Note that the state calculation uses 2026 single-filer rules even when you select tax year 2027, and it does not model pre-tax deductions such as 401(k) contributions, or tax credits, so for other states the result is a federal-only estimate. For the exact bracket figures, see our US tax brackets 2026 vs 2027 page.

Car insurance estimates

Our car insurance estimator starts from published state averages for full-coverage premiums from Insure.com's 2026 data — a national average of $215 a month ($2,578 a year), from $138 a month in Vermont to $333 a month in Louisiana — then adjusts for age, vehicle type and age, coverage level, driving record and annual mileage. Minimum coverage averages $62 a month nationally, and state-minimum liability averages $51 a month. The result is an educational estimate, not a quote: a real premium depends on your exact ZIP code, vehicle, coverage limits, deductible, credit history and driving history. Source: Insure.com — average car insurance cost per month.

Mortgage rates

Our mortgage rates page reports Freddie Mac's Primary Mortgage Market Survey (PMMS), the standard weekly benchmark for US mortgage rates. The survey is based on loan applications submitted through Freddie Mac's Loan Product Advisor, covers conventional, conforming, fully amortizing home purchase loans for borrowers who put 20% down and have excellent credit, and is released every Thursday at 12 p.m. ET. The figures are national averages, not quotes — your actual rate depends on your credit, down payment and loan type. Source: Freddie Mac — Primary Mortgage Market Survey archive.

Review dates

Figures on this site were reviewed against their sources on the dates below. Tax bands and rates change, usually once a year, and we update our tools and reference pages when the official figures are published.

• US federal tax brackets and standard deductions (2026 and 2027) — reviewed November 13, 2026, against IRS Revenue Procedures 2025-32 and 2026-35.
• UK Income Tax, National Insurance and SDLT (2026/27) — reviewed November 13, 2026, against GOV.UK and mygov.scot (Scottish bands).
• US state income tax (California, New York, Illinois, Texas, Florida) — reviewed November 13, 2026, against each state's published 2026 single-filer rates.
• US mortgage rates — reviewed November 13, 2026, against Freddie Mac's Primary Mortgage Market Survey for the week ending November 12, 2026.
• Car insurance state averages — reviewed November 13, 2026, against Insure.com 2026 data.
• Social Security wage base (2026 and 2027) — reviewed November 13, 2026, against the Social Security Administration.

Corrections

Found a figure that looks wrong, or a source that has changed? Please tell us at contact@sumkittool.com and we will check it against the official source and fix it.