Tankless water heater sizing
Tankless Water Heater Size Calculator: Find Your GPM & kW/BTU
Instant result calculator Based on ENERGY STAR + AHRI GPM standards
Methodology: DOE 10 CFR 430 Subpart B Appendix E UEF test (GPM measured at 67°F rise) · ENERGY STAR Gas Tankless Criteria (UEF ≥ 0.95, ≥ 2.8 GPM) · AHRI Directory (third-party GPM/BTU certification) · manufacturers spec analysis (Rinnai RU199i / Noritz NCC199CDV / Bradford White RTGR series).
Your fixtures & fuel
Adjust any input — the GPM and kW/BTU update instantly.
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Recommended tankless
Default for 4-person household, 2 showers, moderate climate, gas. Adjust your inputs on the left for your exact recommendation.
Calculation breakdown
| Peak fixtures GPM | 7.5 GPM |
| Climate derate | none |
| Effective GPM | 7.5 GPM |
| ΔT | 65°F |
| Recommended | 250k BTU |
Formula: BTU = GPM × 500 × ΔT. kW = (GPM × 8.33 × ΔT) / 3412. Cold-climate derate: +25% on GPM. Standard gas sizes: 130k, 150k, 180k, 199k, 250k BTU. Standard electric: 11, 13.5, 18, 24, 27, 36 kW.
Tankless water heaters for your needs
All — tankless models matching your recommended GPM, best-rated first. As an Amazon Associate we earn from qualifying purchases.
How This Calculator Works: GPM + BTU/kW Methodology
Our tankless sizing calculator applies the ENERGY STAR and AHRI GPM standards — the two most widely cited sizing references for on-demand water heaters in the US. The approach: sum your peak simultaneous fixture flow rates, then size a tankless that meets or exceeds that GPM at your climate's inlet temperature.
Step 1: Sum Peak Fixture GPM
Tankless units are sized by flow rate in gallons per minute (GPM), not tank capacity. Sum the GPM of all fixtures that might run simultaneously during peak use (typically morning). Default flow rates per fixture:
—Low-flow shower: 2.0 GPM
—Kitchen sink: 1.5 GPM
—Bathroom sink: 1.0 GPM
—Dishwasher: 1.0 GPM
—Washing machine: 1.5 GPM
Two concurrent showers + kitchen = 5.5 GPM. Add a 1.0 GPM safety buffer for simultaneous peaks.
Step 2: Choose Climate Zone (Inlet Temperature)
Inlet temperature drives the temperature rise (ΔT) the tankless must deliver. The three zones:
Cold (45°F inlet — northern US): ΔT = 75°F, requires 25% more GPM capacity
Moderate (55°F — mid-Atlantic / Pacific NW): ΔT = 65°F, baseline sizing
Warm (65°F — southwest / gulf): ΔT = 55°F, smallest tankless works
Step 3: Convert GPM to kW or BTU
Electric tankless: kW = (GPM × 8.33 × ΔT) ÷ 3412. Round up to standard sizes: 11, 13.5, 18, 24, 27, 36 kW.
Gas (natural or propane) tankless: BTU = GPM × 500 × ΔT. Round up to standard sizes: 130k, 150k, 180k, 199k, 250k BTU.
Propane delivers about 2,516 BTU/ft³ vs natural gas at 1,030 BTU/ft³ — propane tankless needs more fuel volume for the same BTU output.
The Complete Formula
Total GPM = Σ (fixture_count × fixture_gpm) —Inlet °F = climate-based (45/55/65) —ΔT = 120°F − Inlet °F —Cold derate: GPM × 1.25 if climate=cold —Electric kW = (GPM × 8.33 × ΔT) ÷ 3412 —Gas BTU = GPM × 500 × ΔT —Round up to standard size
Why Tankless Sizing Matters
Undersized
- Cold showers mid-stream — the #1 tankless complaint, almost always a sizing issue
- Temperature drops when 2 fixtures run — a 5 GPM unit can't keep up with 2 simultaneous showers
- Inlet temp swings cause output instability — undersized units in cold climates deliver tepid water in winter
Oversized
- Higher upfront cost — a 250k BTU unit costs $400-800 more than a 180k for no benefit in a small home
- Larger gas line required — oversizing may force a 1" gas line upgrade vs 3/4"
- Short-cycling at low flow — a 250k unit on a single bathroom sink may cycle on/off rapidly, shortening heat-exchanger life
5 Common Tankless Sizing Mistakes
- Not derating for cold-climate inlet temperature. A tankless rated for 9 GPM at 35°F rise only delivers 5–6 GPM at 65°F rise. In Minnesota or Maine, derate the manufacturer's GPM by 25-30%.
- Sizing for total fixture count instead of simultaneous peak. A house with 5 fixtures doesn't need a 5 × 2.0 = 10 GPM unit. It needs the GPM of fixtures running AT THE SAME TIME (typically 2 showers + kitchen = 5-7 GPM).
- Trusting the headline GPM number. Manufacturers list GPM at the most favorable inlet temperature (often 35°F or 50°F rise). Check the full GPM-vs-ΔT chart for your climate.
- Sizing electric tankless without checking electrical service. A 27 kW electric tankless needs 3 × 40A 240V breakers — most homes have 200A service that can't support one plus other loads. Often requires panel upgrade ($1,500-3,000).
- Ignoring simultaneous kitchen + bath use. A "small" 4 GPM unit can't handle a shower + dishwasher + bathroom sink all running at once. Always add the 1 GPM safety buffer.
Common scenarios for tankless sizing
If your situation matches one of these, jump straight to a detailed guide:
Frequently asked questions
What size tankless water heater do I need?
Sum the GPM of all fixtures running simultaneously during peak use (typically 2 morning showers + kitchen = 5-7 GPM). For a 4-person household, plan for 7-9 GPM (gas, 180-250k BTU) or 4-5 GPM (electric, 27-36 kW). Use our free tankless calculator.
How many GPM do I need for tankless?
Add the GPM of all fixtures you might run simultaneously during peak use. Shower: 2.0 GPM. Kitchen sink: 1.5 GPM. Bathroom sink: 1.0 GPM. Dishwasher: 1.0 GPM. Washing machine: 1.5 GPM. Two concurrent showers + kitchen = 5.5 GPM. Add a 1.0 GPM safety buffer. In cold climates (groundwater <45°F), derate the manufacturer's GPM by 20-30%.
Electric or gas tankless?
Gas tankless heats faster (180k-199k BTU typical, 8-11 GPM), lower operating cost ($20-40/month), requires gas line + venting. Best for whole-house 3+ person households. Electric tankless (18-36 kW, 4-8 GPM), lower upfront cost ($500-1,500), no venting needed, but slower recovery. Best for point-of-use or small households in warm climates.
What is the cold-climate derate for tankless?
A tankless rated for 9 GPM at 35°F temperature rise only delivers about 6 GPM at 65°F rise. In northern states with groundwater at 45°F or below, derate the manufacturer's GPM by 20-30%. Plan for an extra 25% capacity in cold climates.
Can a tankless water heater replace a tank?
Yes, but requires verification: (1) gas line size — many homes have 1/2" lines that need upsizing to 3/4" for high-BTU gas tankless; (2) electrical — electric tankless needs 2-4 × 40A 240V breakers (most homes need panel upgrade); (3) venting — direct-vent or power-vent required for gas; (4) condensate drain for high-efficiency units.
How do I size for 2 simultaneous showers?
Two showers = 2 × 2.0 GPM = 4.0 GPM baseline. Add kitchen sink (1.5 GPM) and bathroom sink (1.0 GPM) for total 6.5 GPM. For gas: 6.5 × 500 × 65°F = 211,250 BTU → 250k BTU unit. For electric: 6.5 × 8.33 × 65 / 3412 = 10.3 kW → not feasible with standard electric (max 36 kW delivers 8-11 GPM at high ΔT).
What size tankless for a family of 4?
A family of 4 with average use (2 showers + kitchen + laundry) needs a 7-9 GPM gas tankless (180,000-199,000 BTU) or a 4-5 GPM electric tankless (27-36 kW). For cold climates (inlet water below 50°F), add 20-30% to the GPM target.
How long does a tankless water heater last?
Tankless water heaters last 20-25 years with proper maintenance, vs 10-15 years for tanks. Hard water shortens lifespan by 30-50% — annual descaling (vinegar flush) extends life significantly. Look for error codes, reduced flow, or fluctuating temperature as failure signs.
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