What size water heater do I need — and what will it cost to run, repair or replace?
Free, no-signup calculators for homeowners, DIYers and small plumbers planning, running, repairing or replacing a domestic water heater — figure out what size water heater you need (storage gallons + First-Hour Rating, tankless GPM, recovery & reheat time), how much it costs to run (deterministic thermodynamics × your utility rate), how much repairs cost (by component) and how much replacement or installation costs. Every tool runs on the routine you measure and the prices you enter, and shows its formula, a worked example and a reference table.
Example: a busy morning — three showers, a shave, two hand washes and food prep — peaks at about 75 gallons of hot water, so a 50-gallon tank with a First-Hour Rating near 75 (or a tankless sized for the peak GPM) fits. Size by the busiest hour, not the headcount, and round up.
Where do you want to start?
The whole water-heater job, in one hub
Size it → run it → replace or install → repair → choose the type → plan & maintain — a focused hub in US units, in the order a careful homeowner or plumber works.
Sizing
Start here: what size water heater do I need (First-Hour Rating meets your peak-hour demand), peak-hour hot-water demand, first-hour rating, tankless GPM at a temperature rise, recovery & reheat time, size by household, and expansion-tank sizing — from the routine you measure.
Operating Cost & Energy
Budget the running cost: water-heater operating cost (from first-principles thermodynamics + your utility rate), tankless-vs-tank payback, heat-pump / hybrid savings, cost per shower, and energy cost by fuel — on the $/kWh and $/therm you enter.
Replacement & Installation Cost
Budget the swap: water-heater replacement cost, installation cost, tankless installation cost, labor cost, cost by type, and cost by size — on the unit, labor and add-on prices you enter from your own quotes.
Repair Cost
Budget the fix: water-heater repair cost, heating-element, thermostat, anode-rod, temperature-&-pressure relief valve and gas-valve / thermocouple replacement, and a leaking-water-heater repair-or-replace check — parts + labor you enter.
Type & Selection
Choose the type: tank vs tankless, gas vs electric, heat-pump / hybrid, a type selector, and condensing vs conventional gas — labeled cost, efficiency and lifespan comparisons.
Planning & Reference
Plan & maintain: lifespan by type, a maintenance & flush schedule, the 120 °F temperature-setting reference, how to size a water heater (the deterministic method), and an anode-rod & tank-protection reference.
Built for the whole water-heater job — and to stay correct forever
WaterHeaterCalcs gathers the calculations homeowners, DIYers and small plumbers reach for around a domestic water heater — what size you need (First-Hour Rating = 0.70 × storage + recovery; tankless GPM = Σ fixture flow at ΔT = output − inlet), what it costs to run (annual BTU = daily gallons × 365 × 8.33 × ΔT, then ÷ 3412 ÷ UEF for kWh or ÷ 100000 ÷ UEF for therms, × your rate), what a repair or a replacement costs — sizing → energy → cost → repair → selection, in one focused hub, in US units, without signup, with transparent formulas. Every tool shows not just the answer but the underlying formula, a worked example and a reference table.
Because the tools rest on timeless thermodynamics and sizing identities (BTU = gallons × 8.33 × ΔT; recovery = input × efficiency ÷ (8.33 × ΔT); FHR = usable storage + recovery; cost = unit/parts + labor + add-ons, ×(1 + contingency)) and stable, labeled conventions (DOE hot-water use per fixture, gallons + FHR by household, UEF and lifespan by type, inlet temperature by region, cost bands), they stay correct with no maintenance — no live utility rate, no equipment or labor price list, no regional cost index, no product catalog, no plumber directory. Cost tools use the prices you enter from your own quotes and bills; energy tools use your $/kWh and $/therm; labeled cost bands are only a sanity guide. More at Sources & formulas, Methodology and About.
Estimates, not bids. Every result is a planning estimate from your own prices, or a sizing / energy guide — not a bid, an installation procedure, or a gas / combustion / venting, temperature-&-pressure relief, scald / Legionella-medical or code-compliance verdict. Get itemized written quotes from licensed, insured plumbers; confirm your unit’s rated First-Hour Rating, GPM and UEF on its EnergyGuide label and follow the manufacturer’s instructions and local code; round sizing up and leave peak headroom.