Free pool & spa calculators

Pool & Spa Calculators That Show Their Math

Thirty-eight calculators for pool volume, water chemistry, heating, pumps, and cost. Each answer shows its formula and its data date.

Our method

Your test kit in. Your dose out.

01 · Inputs

Test-kit readings, gallons, rates, hours. Nothing estimated behind your back.

02 · Math

Dose, cost, runtime, BTU — each with its formula.

03 · Verify

Test kit, product label, meter data. Baselines are dated.

Calculators

Calculator

Baking soda in ounces, pounds, and 4-lb boxes to reach target TA. Fix alkalinity before touching pH.

Outputs: Baking soda needed (oz) · Baking soda (lb) · 4-lb boxes

Calculator

Gallons per backwash cycle and per month from filter GPM, minutes, and frequency, plus water cost.

Outputs: Gallons per backwash (gal) · Gallons per month (gal) · Monthly water cost (USD)

Calculator

Borax in pounds and 4-lb boxes for 30-50 ppm borates, plus cost. Borates buffer pH but never replace chlorine.

Outputs: Borax (20 Mule Team) (lbs) · 4-lb boxes · Borax cost (USD)

Guides

Article

Evaporation and Pool Covers: calculate daily water loss, compare cover performance, and estimate realistic water savings in 2026.

Article

Use this Pool Build Budget Guide for 2026 to estimate an inground pool, decking, fencing, heater installation, and site-specific costs.

Saved pool records

Free to use. Sign in to save your pool's numbers, compare what-if scenarios, and print the report.