Free calculators
that show their math.

Mortgage, tax, investing, business, health and home-energy calculators. Each one runs in your browser, shows the formula it uses, and links to a plain-English guide.

Runs in your browserNo signupFormula shownReviewed September 2026

Personal Finance

Mortgage payoff, refinancing, rent vs. buy, debt payoff order, emergency funds and compound growth.

8 calculators

Tax

The new 2026 overtime, tips, car loan and senior deductions, self-employment tax and quarterly payments.

5 calculators

Retirement

Your FIRE number, Social Security claiming age, required minimum distributions, Roth vs. traditional and HSAs.

7 calculators

Business

Software ROI, ad-spend returns, project estimates and what your time is really worth.

5 calculators

Health

BMI, body fat, waist-to-hip and waist-to-height ratios, and daily calorie and macro targets.

5 calculators

Home Energy

Solar payback, home battery backup, electric vs. gas car running costs and household carbon.

4 calculators

All Calculators

Compound Interest

Project savings growth with monthly contributions

Mortgage Payoff

See the interest and years extra payments save

Income Tax Estimator

Estimate 2026 federal tax from the IRS brackets

Debt Consolidation

Compare total interest before and after consolidating

Refinance Break-Even

Find the month a refinance pays back its closing costs

Rent vs Buy

When owning beats renting, year by year

Debt Snowball vs Avalanche

Which payoff order saves more and finishes sooner

Emergency Fund

How many months of expenses to keep in cash

No Tax on Overtime

How much of your overtime pay is now tax-free

No Tax on Tips

Your tip income deduction and tax saved

Car Loan Interest Deduction

What your new-car loan interest saves at tax time

Senior Deduction

The extra $6,000 deduction for people 65 and over

Self-Employment Tax

Your 1099 tax bill and quarterly payments

401(k) Paycheck

What a 401(k) contribution and match do to each paycheck

401(k) Early Withdrawal

What you keep after the 10% penalty and tax, or a loan instead

FIRE Number

How much you need to retire early, and when you get there

Social Security Break-Even

Claiming at 62, 67 or 70: which pays more over your life

RMD Calculator

Your required minimum withdrawal this year

Roth vs Traditional

Which 401(k) or IRA leaves you more after tax

HSA Growth

Your HSA as a tax-free retirement account

SaaS ROI & TCO

Weigh a software subscription against the time it saves

ROAS Calculator

Check whether ad spend is profitable after margin

Project Timeline (PERT)

Turn best, likely and worst cases into an estimate

Real Hourly Rate

Your true hourly rate after commute and unpaid time

AI Productivity

Estimate the value of hours an AI tool saves

BMI Calculator

Body mass index with the WHO adult categories

Macro Planner

Daily calories and protein, carb and fat targets

Body Fat %

U.S. Navy tape-measure body fat estimate

Waist-Hip Ratio

Waist-to-hip ratio against WHO risk thresholds

Waist-to-Height Ratio

A quick belly-fat risk check, compared with BMI

EV Savings

Compare fuel costs of an electric and a gas car

Solar Payback

Years until home solar pays for itself

Carbon Footprint

Estimate household CO2 from energy and travel

Home Battery Backup

The battery size you need and how long it runs

Start with a question

Common money, business and health questions, the short answer, and where to run your own numbers.

Is paying $100 extra on my mortgage worth it?

Extra principal cuts total interest and ends the loan early. Enter your balance and rate to see how many months and dollars it saves.

Open the calculator Guide: Extra $100 a Month on Your Mortgage: How Much It Saves

Will consolidating my debt actually save money?

Only if total interest falls, not just the monthly payment. A longer term can make a lower rate cost more overall.

Open the calculator Guide: Debt Consolidation Loans: Why a Lower Payment Can Cost More

Is my ad spend actually profitable?

Break-even ROAS is 1 divided by your gross margin. Revenue below that multiple of spend loses money on every sale.

Open the calculator Guide: What Is a Good ROAS? How to Find Your Break-Even ROAS

Do I need a waist measurement as well as BMI?

BMI cannot tell where weight is carried. Waist-based ratios add that signal, which is why clinicians often use both.

Open the calculator Guide: Waist-to-Height Ratio vs BMI: What Each One Tells You

How long until home solar pays for itself?

Divide the net installed cost by the yearly bill savings. Systems installed from 2026 no longer get the 30% federal credit.

Open the calculator Guide: Section 25D Solar Tax Credit Ended: What's Left in 2026

Should I pay off debt smallest-first or highest-rate first?

Highest rate first (avalanche) always costs less interest. Smallest first (snowball) can win if early wins keep you paying.

Open the calculator Guide: Debt Avalanche vs. Snowball: Which Payoff Method Is Better?

How much of my overtime pay is tax-free now?

Only the extra half of time-and-a-half is deductible, up to a yearly cap, and the deduction shrinks at higher incomes. It lowers income tax, not Social Security or Medicare.

Open the calculator Guide: "No Tax on Overtime" 2026: What the Deduction Covers

Should I claim Social Security at 62, 67 or 70?

Waiting pays a bigger check for life. The break-even age, often around 80, tells you how long you need to live for waiting to come out ahead.

Open the calculator Guide: Social Security Claiming Age: The Break-Even Math

How much will a 401(k) contribution cut my paycheck?

Less than you put in. A pre-tax contribution lowers your income tax, so on a $75,000 salary 6% ($173 a paycheck) cuts take-home pay by about $126. Social Security and Medicare don’t change.

Open the calculator Guide: 401(k) Employer Match: How It Works and How People Miss It

How these calculators work

No black boxes. Every result can be checked by hand from what the page shows you.

Standard, published formulas

Loan amortization, the IRS tax brackets, the World Health Organization BMI and waist-to-hip cut-offs, the U.S. Navy body-fat equation. Each page names its formula and its source.