Calculators

Skip the guesswork and the sticky-note math. These calculators handle the numbers DIY projects hinge on most often: how much paint to buy, how many bags of concrete a slab or footing needs, and what size air conditioner actually fits a room. Each one works the way our guides do — plain English, real formulas you can check yourself, and results sized to an actual hardware-store purchase, not a theoretical decimal. Enter your room's numbers and get a straight answer: paint rounded to the nearest quart, concrete bags rounded up so you're never short on pour day, BTU/hr sized against the same room-AC guidelines professionals use. No sign-up, no app download — just open a calculator, plug in your numbers, and get a number you can act on at the store. Each calculator also explains its own math in plain English further down the page, with a worked example and the exact source table or formula behind it, so you're never just trusting a black box. If a number looks off for your specific situation, you'll be able to see exactly why and adjust from there. We'll add more calculators here as new guides call for them.