Why TermCalc
- Free forever — every tool works without an account, a paywall or a watermark.
- Private by design — everything runs in your browser; no invoice data or numbers are ever uploaded.
- Built for small business — payment terms, working days, VAT and sales tax, margins and markups: the maths you actually do every week, in one place.
Start with the most popular tool: the invoice due date calculator — tell it the invoice date and terms, get a calendar answer in seconds. Sending the invoice next? The invoice generator prints a clean PDF in one click.
The maths most people get wrong
Each calculator on this site exists because a specific mistake happens constantly in real businesses:
- "Net 30" read as working days when it means calendar days — a 10-day difference that turns a healthy cash flow into a crunch. The due date calculator supports both, and writes the exact calendar date.
- Markup quoted when margin was meant — a "50% markup" is only a 33.3% margin, and pricing decisions built on the wrong one quietly undercharge. See both at once on the profit margin calculator.
- VAT removed by subtraction — taking 20% off a gross figure understates the tax, because VAT was calculated on the smaller net. Divide by 1.2 instead; the VAT calculator shows both routes.
- Stacked discounts added — 20% off then 25% off is 40% off, not 45%. The discount calculator multiplies correctly.
Every page explains its formula and links the primary source it came from — OPM for the federal holiday calendar, GOV.UK for VAT rates, FTC rules for pricing claims — so you can check our working, not just our answer.
How to pick the right calculator
Seventeen tools is a lot to hold in your head, so here is the entire set mapped to the job in front of you. Find the sentence that matches your situation, then follow the link. Nothing is hidden behind the grid — if a task is date-based or money-based, one of these covers it.
| What you want to do | Use this calculator |
|---|---|
| Work out the due date for a single invoice (Net 30/60/90, EOM or custom terms) | Invoice Due Date Calculator |
| Count working days between two dates, or add a set number of working days | Business Days Calculator |
| Get the days between two dates, or add and subtract from a date, all in one place | Date Calculator |
| Add a fixed number of days (30, 60 or 90) to a date and read off the result | Add Days to a Date |
| Check whether a chosen date is a US federal holiday in 2027 | US Federal Holidays 2027 |
| Create, total and print a clean PDF invoice for a client | Invoice Generator |
| Calculate due dates for a whole register pasted from a spreadsheet or CSV | Bulk Invoice Due Dates (Excel) |
| Turn daily clock-in and clock-out times into weekly hours and gross pay | Time Card Calculator |
| Log hours by project, flag what is billable and export the result as CSV | Timesheet Calculator |
| Work out overtime pay from hours worked and a base rate | Overtime Pay Calculator |
| Convert an hourly rate into annual, monthly or weekly pay — and back | Hourly to Salary Calculator |
| Add US sales tax to a price, or pull the tax back out of a total | Sales Tax Calculator |
| Add or remove UK and EU VAT at 20%, 5% or 0% | VAT Calculator |
| Find gross margin and markup from a revenue figure and a cost | Gross Margin Calculator |
| Find margin, cost and profit from a selling price, or price to a target margin | Profit Margin Calculator |
| Set a selling price from a cost and a chosen markup | Markup Calculator |
| Apply a percentage off a price, or stack two discounts in the right order | Discount Calculator |
Worked examples you can check
Every figure below was produced by the same engine the calculators run, so you can reproduce it by hand and confirm the tool agrees with your own working. These are the six cases that generate the most support questions.
| Scenario | Inputs | Result |
|---|---|---|
| Net 30 calendar due date | Invoice date 3 Nov 2026, terms Net 30 | Due 3 Dec 2026 |
| Calendar days versus business days | 30 days from 3 Nov 2026 | 3 Dec (calendar) / 15 Dec (weekends skipped) / 17 Dec (weekends and US federal holidays skipped) |
| Markup is not the same as margin | Cost $100, markup 50% | Price $150, margin 33.33% |
| Removing VAT the correct way | Gross $120 including 20% VAT | Net $100, VAT $20 (divide by 1.2) |
| Stacked discounts do not add up | $200 less 20% then less 25% | $120 — 40% off, not 45% |
| Overtime at time and a half | 45 hours at $20 per hour | $950 (40 × $20 + 5 × $30) |
Notice the second row: the same 30-day term lands on three different dates depending on how you count. Calendar days give 3 December. Skipping weekends only gives 15 December. Skipping weekends and the two US federal holidays that fall inside the window — Veterans Day on 11 November and Thanksgiving on 26 November — gives 17 December. That is a 14-day spread on a single payment term, which is precisely why the date tools let you choose the convention rather than silently picking one for you.
How these calculators are built and checked
A calculator is only worth using if you can trust the number it prints, so every tool on TermCalc follows the same three rules.
- The formula comes from a first-party source. Where a rate or a rule is defined somewhere official — the OPM federal holiday schedule, GOV.UK VAT rates, the Small Business Administration's explanation of payment terms — that is what the page implements, and the source is linked so you can read it yourself.
- The engine is tested. The date and money routines that do the arithmetic run against an automated test suite, so a change that breaks end-of-month handling or overtime rounding fails before it ever reaches the site.
- No number on the page is hand-typed. The worked examples above were generated by the code the calculators use, which is why they reproduce exactly to the day and the cent.