Calculator
Water Heater Sizing Calculator for Contractors
How to Size a Water Heater for Any Home
Undersized water heaters are one of the most common customer complaints after installation. The problem is that most contractors size by household count alone — but a family of 4 that takes back-to-back morning showers needs a completely different first-hour rating than one that spreads usage throughout the day. This water heater sizing calculator uses the DOE peak hour demand method to recommend tank or tankless units based on actual fixture usage, fuel type, and climate zone. It calculates the first-hour rating for tank models, the GPM and BTU requirements for tankless models, and estimates annual operating costs, so you can help homeowners compare options during the sale. Use the results alongside TackonFSM’s quoting tools to build accurate proposals on-site.
Water Heater Sizing Calculator
Size tank or tankless water heaters by peak demand, first hour rating, BTU input & estimated annual cost
Household
Peak Hour Fixtures (What runs during your busiest hour?)
🚿 Showers20 gal / 2.5 GPM each
0
🛁 Baths36 gal / 4.0 GPM fill
0
🚰 Kitchen Sink4 gal / 1.5 GPM
0
🧼 Bathroom Sink2 gal / 1.0 GPM
0
🍽️ Dishwasher6 gal / 1.5 GPM
0
🧺 Clothes Washer7 gal / 2.0 GPM
0
Peak Hour Demand: — gal | Simultaneous Flow: — GPM
System Parameters
Recommended Water Heater
—
—
Peak Hour Demand—
First Hour Rating (FHR) Needed—
Simultaneous Flow Required—
Temperature Rise—
BTU Input Required—
Fuel Type—
Recovery Rate—
Estimated Annual Energy Cost—
Tank Size Options for Your Demand
| Tank Size | Est. FHR (Gas) | Est. FHR (Electric) | Fits Your Peak? |
|---|
FHR = First Hour Rating. Choose a tank where FHR ≥ your peak demand. Gas heaters recover faster than electric.
Tankless Sizing Guide by Flow Rate
| Flow Rate (GPM) | Min BTU (Gas) | Min kW (Electric) | Typical Use |
|---|---|---|---|
| 2–3 | 60,000 | 14 kW | 1 shower OR 1 sink |
| 3–5 | 100,000 | 18 kW | 1 shower + 1 sink |
| 5–7 | 150,000 | 24 kW | 2 showers OR shower + appliance |
| 7–9 | 180,000 | 28 kW | 2–3 fixtures simultaneous |
| 9–11 | 199,000 | 32 kW | Whole-house high demand |
Gas tankless max out at ~199,000 BTU. For demand above 11 GPM, install multiple units or a commercial system.
Quick Reference: Tank Size by Household
| Household | Gas Tank | Electric Tank | Tankless GPM |
|---|---|---|---|
| 1–2 people | 30–40 gal | 40–50 gal | 3–4 GPM |
| 2–3 people | 40–50 gal | 50–60 gal | 4–6 GPM |
| 3–4 people | 40–50 gal | 50–65 gal | 5–7 GPM |
| 5–6 people | 50–75 gal | 65–80 gal | 7–9 GPM |
| 7+ people | 75–100 gal | 80–100 gal | 9+ GPM (multi-unit) |
Calculation basis: Peak hour demand per DOE sizing method. BTU = peak GPM × temperature rise × 500.4. Recovery rates: gas ~40 GPH, electric ~23 GPH, heat pump ~10 GPH (varies by model). Annual cost uses avg U.S. energy rates: gas $1.20/therm, electric $0.14/kWh, propane $2.50/gal. Tank FHR estimates assume standard efficiency (gas 0.60 UEF, electric 0.93 UEF, heat pump 3.0 COP). Always verify with manufacturer specs and local codes.
Frequently Asked Questions
A family of 4 typically needs a 40–50 gallon gas water heater or a 50–65 gallon electric water heater. The key is the first hour rating (FHR) — not just the tank size. If your family takes back-to-back showers in the morning, you need an FHR of at least 60–70 gallons. Gas heaters recover faster (about 40 gallons per hour) so they can use a smaller tank than electric (about 23 gallons per hour). Use this calculator's fixture selector to get a precise recommendation based on your actual morning routine.
First hour rating (FHR) measures how many gallons of hot water a tank water heater can deliver in one hour, starting with a full tank. It combines the tank capacity with the recovery rate. A 50-gallon tank with a fast gas burner might have an FHR of 80+ gallons, while a 50-gallon electric tank might only deliver 60 gallons in that first hour. The DOE requires FHR on every EnergyGuide label. When sizing, your FHR should be equal to or greater than your peak hour demand — the hour when your household uses the most hot water.
Tankless water heaters are sized by flow rate (GPM) and temperature rise, not gallons. Add up the flow rates of all fixtures you want to run simultaneously — for example, one shower (2.5 GPM) plus a kitchen sink (1.5 GPM) equals 4.0 GPM. Then calculate the temperature rise: subtract your inlet water temperature from 120°F. In cold climates with 45°F inlet water, that's a 75°F rise. The BTU formula is: GPM × temperature rise × 500.4. A gas tankless unit maxes out at about 199,000 BTU, which delivers roughly 8–9 GPM at a 50°F rise or 5–6 GPM at a 75°F rise.
Tank water heaters cost less upfront ($800–1,500 installed) and handle multiple simultaneous uses without performance drops. They're the right choice for most families. Tankless units cost more ($2,000–4,500 installed) but save 15–30% on energy by not keeping a tank hot 24/7. They also last 20+ years vs 10–12 for tanks. Tankless makes sense for small households, vacation homes, or when space is tight. But in cold climates, a single tankless unit may not deliver enough GPM for a large family running multiple fixtures at once — you'd need a second unit or a high-BTU model.
The standard recommendation is 120°F. This is hot enough for showers and cleaning while reducing scalding risk, especially in homes with children or elderly residents. Setting it to 140°F wastes energy (about $36–60/year extra) and creates scalding danger — water at 140°F can cause third-degree burns in 5 seconds. The only reason to go higher is if you have a dishwasher without a built-in booster heater, which needs 140°F for sanitization. Most modern dishwashers have built-in boosters.
Launch with ease
Ready to Simplify Your Field Service Operations?
Start your free 14-day trial today. No credit card, no setup fees — just powerful tools built for contractors.