Who builds this
PitStopCalc is built and maintained by Alex, who spent fifteen years in semi and heavy equipment repair before writing a line of code. Every calculator here started as math done by hand in the shop — usually with somebody waiting on the number. The tools available online gave you a result with no formula and no way to check it. These show the work.
Why this site exists
Fuel cost, gearing, towing limits, and engine displacement all come down to straightforward math — but finding a calculator that shows its work, instead of just spitting out a number, is harder than it should be. PitStopCalc lays out the formula behind every result right on the page, so you can see exactly how the number was reached.
What you'll find here
Each calculator uses standard, published automotive and trucking-industry formulas — the same references a diesel tech, fleet mechanic, or gearhead would work from. Every calculator also documents its formula and answers common questions specific to that topic, right on the page.
Free, no sign-up, ad-supported
Every calculator is free to use, with no account required. The site is supported by advertising rather than subscriptions or paywalls, which is why you'll see a couple of ad placements on each page — kept clear of the calculator forms so they don't get in the way of the tool.
Accuracy first
Formulas are checked against standard industry references. That said, every result here is an estimate for planning purposes — always confirm critical figures like towing capacity, payload, and gearing against your vehicle or equipment manufacturer's specs. See the Disclaimer for details.
How each calculator is checked
Showing the formula only helps if the formula is right. Every calculator here goes through the same checks before it is published, and again whenever it changes:
- Hand-calculated answers. Every figure in a worked example or reference table is worked out by hand first, then read back off the live calculator. If the two disagree, one of them is wrong, and it gets found before the page goes out.
- Every table row can be reproduced. Each row shows the inputs that produce it, so you can type them in and get the same result. A row that can't be reproduced is treated as an error, even when its arithmetic is right.
- Primary sources, linked. Legal limits and published constants come from the source itself: federal regulations on eCFR, FHWA size and weight guidance, and standards from SAE International, ASTM, API and the U.S. Tire Manufacturers Association. Each calculator page links them in its "Where these numbers come from" section. A source that can't be opened and checked isn't cited.
- Checked against published figures. Where an authority publishes its own worked numbers, the calculator is run against them. The stopping-distance model reproduces the FMCSA CDL manual's 55 mph breakdown to within 2 feet.
- Every unit and every language. Each calculator is tested in US and metric units, and switching back and forth must never change a value you typed. Corrections are made in all six languages at once, never in English alone.
Corrections
Mistakes happen, and the honest response is to fix them in the open. In August 2026 all eleven calculators were re-audited from scratch. Six errors turned up and were fixed, among them an hours-of-service result that didn't account for the required 30-minute break, a stopping distance based on a passenger car rather than a loaded truck, and a towing limit that left out the trailer's tongue weight. If a result looks wrong to you, send the inputs you used. Reports of a wrong number are dealt with before anything else.
Questions or a calculator request?
Reach out any time — see the Contact page.