Calculator Methodology

Last reviewed: October 2026

Every calculator on The Tools Kit gives an estimate for planning. It is not tax, legal, medical or financial advice. It does not replace an accountant, a lender or a doctor. This page shows how each calculator works, so you can decide how far to trust the result.

For each one we list the formula, any reference figures it uses and where they come from. Where the code names a source, we name it too. Where it does not, we say so. Our Editorial Policy explains how we keep this content up to date.

Limits that apply to every calculator

• Results are rounded, usually to the cent, penny or whole unit shown.
• Where a calculator uses a reference figure or a rule of thumb, its section below says so.
• Always confirm a medical, legal or money decision with the right professional.

Everyday money

Discount Calculator

The sale price is the price times (1 − discount). A second discount is applied to the already reduced price. So 20% then 10% off is 28% off in total, not 30%.

Tip Calculator

The tip is the bill times the tip rate, rounded to the cent. When you split the bill, every share is in whole cents and the shares add up to the exact total. Any spare cent goes to the first shares. The tip is worked out on whatever amount you type, before or after tax.

Percentage Calculator

Three formulas: X% of Y is (X ÷ 100) × Y. X as a percent of Y is (X ÷ Y) × 100. Percent change is (new − old) ÷ old × 100. Results show up to two decimal places.

Pay and hours

Time Card Calculator

Hours are time out minus time in, with optional rounding, minus unpaid breaks. Overnight shifts are handled. Federal (FLSA) mode pays overtime on hours over 40 in a week. California mode pays 1.5× after 8 hours a day and 2× after 12. It does not apply California's seventh-day rule. Pay is before tax.

Hourly to Salary Calculator

Hourly pay times hours per week times 52 gives the yearly salary. Other periods use 52 weeks, 26 fortnights, 24 half-months or 12 months. The adjusted column treats your holidays and vacation days as unpaid. All figures are before tax.

Salary to Hourly Calculator

This runs the same math the other way. Yearly salary divided by (hours per week × 52) gives the hourly rate. For example, $60,000 at 40 hours a week is $28.85 an hour.

Overtime Calculator

Hours up to your threshold (40 by default) are paid at your rate. Hours above it are paid at 1.5× or 2×. There is one threshold and one multiplier per run. Pay is before tax.

Budgets, business and planning

Moving Cost Calculator

We measure the straight-line distance between the two cities, not the driving distance. Then we look up a price range for your home size. The table has prices at set distances from 50 to 3,000 miles, and we fill in the gaps between them. The price table is our own estimate of typical market prices. The code does not cite an outside source for it. Container and freight prices are fixed fractions of the full-service range. Season, packing, stairs and storage are not included.

This is the one calculator that uses our servers. City search goes through our server to Photon (OpenStreetMap). To see your estimate, you must give a name, email and US phone number. We store these with your quote request. They are not sold or passed to moving companies.

SaaS Cost Calculator

Five products with list prices in US dollars, last checked July 1, 2026. Cost is the per-seat price times seats. The 3-year cost uses the annual-plan price times 3. Tax, discounts and price rises are not included.

Invoice Generator

Each line is quantity times unit price, rounded to the cent. A percent discount comes off the subtotal, then tax is added to what is left. Amounts are shown in US dollars.

Wedding Cost Calculator

Your budget is split by fixed shares, for example 45% for venue and catering. The starting budget ($30,000 in the US and Canada, £20,000 in the UK) is a round planning figure we chose. It is not a survey average and is not tied to any survey or year. Changing country changes the currency sign only. There is no currency conversion.

Baby Cost Calculator

Your budget is split by fixed shares, for example 20% for gear. The starting figure ($13,000 in the US and Canada, £10,000 in the UK) is a round planning figure we chose for first-year essentials, not a published average. Childcare is not included.

Vacation Budget Calculator

Your budget is split by fixed shares: 30% flights, 30% stays, 20% food, 10% activities, 5% shopping and 5% buffer. The starting figure is an example for one person on a one to two week trip abroad. It is not a statistic.

Health, dates and school

BMI Calculator

BMI is weight in kg divided by height in metres squared. In pounds and inches it is (lb ÷ in²) × 703. Categories are under 18.5, 18.5 to 24.9, 25 to 29.9, and 30 or more. There is no change for age or sex, and BMI does not measure body fat.

Calorie Calculator

We use the Mifflin-St Jeor equation for resting calories (BMR). Then we multiply by an activity factor from 1.2 to 1.9. Weight-loss targets are 250 or 500 calories below that, and the gain target is 500 above. This is a general guide, not medical advice.

Due Date Calculator

From the first day of your last period, we add 280 days. From conception, we add 266 days. From an IVF transfer, we add 263 days for a day-3 embryo or 261 for a day-5 embryo. Your doctor or scan may give a different date.

Age Calculator

We count whole years, then whole months, then the days left over. If a month is shorter, the day is moved to that month's last day. A 29 February birthday turns a year older on 1 March in other years.

Days Calculator

Days between two dates counts the start date but not the end date. Business days count Monday to Friday, including both dates. Public holidays are not removed.

GPA Calculator

GPA is total grade points times credits, divided by total credits. We use an unweighted 4.0 scale, so A and A+ are both 4.0. There is no extra weight for honors or AP classes.

Grade Calculator

Your average is each score times its weight, divided by the total weight. The final-exam mode solves for the score you need: (target − current × current weight) ÷ final weight. Letter grades use a common US scale: 93 and above is an A, 90 to 92 is an A-, and below 60 is an F.

Where calculations run

All of these calculators run in your browser except one. The numbers you type are not sent to us. The exception is the Moving Cost Calculator, which uses our servers as described above.

Something look wrong?

Tax rates and limits change every year. If a figure here is out of date, or a formula looks wrong, email us at thetoolskit.co@gmail.com. We will check it against the official source and correct it.