RunBuzz

Free Running Calculators & Tools

Coach-built tools for runners. No ads, no sign-up, no popups. Every calculator runs in your browser — your inputs stay on your device, and the result URLs are shareable with a training partner or coach.

VDOT Calculator

Your training paces

Enter a recent race. Get your VDOT fitness score, race-equivalent finish times at every distance, and the exact paces to hit on easy, marathon, threshold, interval, and repetition runs.

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Pace Calculator

Pace, time, or distance

Enter any two of distance, time, or pace — get the third instantly. Works with miles or kilometers, preset race distances, and shareable result URLs.

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Race Time Predictor

Every distance, one input

Enter a recent race and see predicted finish times from 1500m through the marathon. Uses the Jack Daniels VDOT model — more realistic than Riegel for long-distance predictions.

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Heart Rate Zone Calculator

Your 5 training zones

Enter your age (or your measured max HR) and get your five training zones. Supports both % Max HR and Karvonen (HR Reserve) when you know your resting heart rate.

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Running Training Load Calculator

Plan weekly mileage safely

ACWR-based weekly mileage planner. Flags injury risk on your planned next week and builds a 12-week progression to your goal — with deload weeks built in. Smarter than the 10% rule.

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Race Fueling Calculator

Half marathon & marathon

Personalized carb, fluid, and sodium targets for race day, plus carb-loading and recovery plans. Built on the latest peer-reviewed sports-dietitian guidance.

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Which tool should I use?

Short answer: use whichever one matches the question you have right now.

  • "What should I train at today?" → VDOT calculator. It gives you easy, marathon, threshold, interval, and repetition paces based on a recent race.
  • "How fast did I just run?" or "What pace do I need for my goal?" → pace calculator. Pure pace/time/distance arithmetic with mile/km conversion.
  • "Given my recent 5K, what can I run in the half?" → race time predictor. Takes one race and projects every other distance.
  • "What heart rate should I hit on easy days?" → heart rate zone calculator. Five training zones from your age or measured max HR.
  • "Is my planned mileage going to get me hurt?" → training load calculator. ACWR-based weekly mileage planner with deload weeks built in.
  • "How many gels do I need on race day?" → race fueling calculator. Personalized carbs, fluid, and sodium for your half marathon or marathon.

Why these tools

Most online running calculators do one thing in isolation and bury you in ads. These tools share a common math engine, they cross-link to each other, and they are maintained by a working running coach — Steve Carmichael, RRCA/USATF certified.

They run entirely in your browser. Your race time doesn't leave your device, and the URL you copy is the complete state of the tool, so you can share it with anyone or bookmark it for later.

The math behind the tools

All three tools share a core implementation of Jack Daniels' VDOT framework:

  • VO₂ cost of running:Daniels&Gilbert formula relating velocity to oxygen demand.
  • Fraction of VO₂max sustainable: a two-exponential curve that captures how much of your maximum aerobic capacity you can hold for a given duration.
  • VDOT score:your race velocity's VO₂ cost divided by the fraction of max you're sustaining for that duration.

This framework is why predicting a marathon from a 5K gives a realistic (and sometimes sobering) answer — it knows you can't hold 95% of your VO₂max for 3 hours, even if you can hold it for 20 minutes.

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