Calculators that get the details right
Small tools for people building habits and running challenges. Correct date math (Day 1 counts), research-backed numbers (with sources), and answers to the questions the generic sites skip.
75 Hard Tracker (free, online & printable) All 75 days × 5 tasks with auto-save in your browser — no signup — plus a clean printable wall checklist. 75 Hard End Date Calculator Your exact finish date and milestone days for 75 Hard, 75 Soft, or 30/100-day variants — plus restart math and the official rules. Habit Formation Calculator How long until your habit becomes automatic, adjusted for difficulty and consistency — based on the UCL study behind the "66 days" figure, not the 21-day myth. Running Streak Calculator Your exact current streak day and the calendar dates of every milestone from 30 to 1,000 days — with the official USRSA one-mile rule. Gallon of Water a Day Schedule An hour-by-hour drinking schedule built around your wake time, bedtime, and bottle size — front-loaded so it doesn't wreck your sleep. Deload Week Calculator The exact dates of your next deload weeks from your block start date and frequency — plus how to actually deload (volume vs intensity).Why another calculator site?
Because most of the ones that rank get the basics wrong: they add 75 days instead of 74 to a challenge start date, or repeat the debunked 21-day habit rule with no source. Each tool here states its method, cites its sources where research is involved, and answers the follow-up questions people actually have.
Want it on paper? The 75 Hard Complete Kit is a 9-page printable pack — full 75-day wall tracker, cut-out daily cards, water schedule, photo log and a restart plan. See what's inside →