TL;DR
Gas tankless formula: BTU = GPM ×500 ×ΔT. For 7 GPM at 65°F rise = 227,500 BTU — 250k BTU. Standard sizes per ENERGY STAR: 130k, 150k, 180k, 199k, 250k BTU.
Why gas tankless is the most common whole-house choice
Gas tankless water heaters use the formula BTU = GPM ×500 ×ΔT, where 500 is a constant that converts GPM ×°F to BTU/hr. Per DOE 10 CFR 430 Subpart B Appendix E, GPM is measured at a 67°F rise — the federal test condition.
Worked example: 7 GPM at 65°F rise: 7 ×500 ×65 = 227,500 BTU — round up to 250k BTU. Standard residential sizes per ENERGY STAR: 130k, 150k, 180k, 199k, 250k BTU (432 certified gas tankless models in the US market as of June 2026).
Top certified models (manufacturer spec analysis):
- Noritz NCC199CDV: 199k BTU, 5.5 GPM at 67°F rise, UEF 0.98, NG/LP dual-fuel. Most efficient top-tier.
- Rinnai RU199iN: 199k BTU, ~11 GPM at 35°F rise / ~6 GPM at 65°F rise, UEF ~0.97, NG.
- Bradford White RTGR150N1: 150k BTU, 4.3 GPM at 67°F rise, UEF 0.98, NG.
- Kiturami KGWH-150JFN: 150k BTU, 4.3 GPM, UEF 0.98, NG.
Cold-climate derate: Per Fixr 2025, a 4.5 GPM unit at 52°F inlet drops to ~3.5 GPM at 35°F inlet (-22%). Same unit at 72°F inlet reaches 6.8 GPM (+51%). Every 10°F drop reduces output 0.5–0.7 GPM, or 10–15% capacity loss.
Top picks for this size
EcoSmart ECO 11 Electric Tankless Water Heater, 13KW at 240 Volts with Patented Self Modulating Technology #ad
★★★★ 4.1 · 5K+ · $244
Amazon
EcoSmart ECO 27 Electric Tankless Water Heater, 27 KW at 240 Volts, 112.5 Amps with Patented Self Modulating Technology, 17 x 17 x 3.5, White #ad
★★★★☆ 4.5 · 3K+ · $475
Amazon
Ecosmart ECO 24 24 KW at 240-Volt Electric Tankless Water Heater with Patented Self Modulating Technology, 17 x 17 x 3.5 #ad
★★★★☆ 4.7 · 337+ · $501
Amazon
Rheem 18kW 240V Tankless Electric Water Heater, Gray #ad
★★★★ 4.2 · 4K+ · $438
Amazon
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray #ad
★★★★ 4.1 · 4K+ · $546
Amazon
Rheem 11kW 240V Tankless Electric Water Heater #ad
★★★★☆ 4.7 · 4K+ · $255
Amazon
Rheem 240V Heating Chamber RTEX-13 Residential Tankless Water Heater, GRAY #ad
★★★★ 4.4 · 1K+ · $273
Amazon
Stiebel Eltron Tankless Water Heater – Tempra 29 Plus – Electric, 28.8kW, On Demand Hot Water, Eco, White, Made in Germany #ad
★★★★ 4.2 · 446+ · $695
Amazon
Stiebel Eltron Tankless Water Heater – Tempra 24 Trend – Electric, 24kW, On Demand Hot Water, Eco, White, Made in Germany #ad
★★★★ 4.2 · 90 · $620
Amazon
Rinnai V53DeN Natural Gas Tankless Water Heater - 5.3 GPM, Outdoor Installation #ad
★★★★ 4.2 · 230+ · $640
Amazon
Rinnai RE199iN Non-Condensing Natural Gas Tankless Water Heater, 199,000 BTU, 9.8 GPM On-Demand Hot Water, Indoor Whole House Installation #ad
★★★★ 4.2 · 39 · $1279
Amazon
EcoSmart ECOS 27 Tankless Electric Water Heater, 27 kW at 240 Volts, 112.5 Amps with Self Modulating Technology #ad
★★★★★ 5 · 13 · $420
Amazon
Rheem RTG-84XLN-3 High Efficiency Non-Condensing Outdoor Tankless Natural Gas Water Heater 8.4 GPM, 180,000 BTUs #ad
★★★★ 4.2 · 27 · $953
Amazon
Rheem RTG-70XLN-3 High Efficiency Non-Condensing Outdoor Tankless Natural Gas Water Heater, 7.0 GPM, Gray #ad
★★★★ 4.2 · 63 · $804
Amazon
Electric Tankless Water Heater SIVUATEK 24kW, 240V - On-Demand Hot Water for Whole House - ETL Certified, Self Modulating - Compact Design for Easy Installation - for 4 Point of Use Hot Water #ad
★★★★ 4.3 · 250+ · $275
Amazon
Ranein 18kW Electric Tankless Water Heater, 240V On Demand Instant Endless Water Heater with LED Temperature Display, Control Knob, Space Saving Design, Up to 4.3 GPM for Whole House Water #ad
★★★★ 4.4 · 361+ · $208
Amazon
Rheem 6.5kW 240V Point of Use Thermostatic Tankless Electric Water Heater #ad
★★★★ 4.2 · 4K+ · $189
Amazon
NEW Rinnai RX199iN Condensing Smart Sense Natural Gas or Propane Tankless Water Heater, Indoor or Outdoor Water Heater, Up to 11.1 GPM, 199,000 BTU #ad
★★★★ 4.2 · 137+ · $1533
Amazon
NEW Rinnai RXP199iN Smart Sense Condensing Tankless Water Heater, 199,000 BTU, Up to 11.1 GPM, Natural Gas or Propane, Indoor/Outdoor Installation, Built-In Recirculation Pump #ad
★★★★ 4.2 · 116+ · $1861
Amazon
Ranein Propane Gas Tankless Water Heater, Indoor Max 3.6 GPM, 80,000 BTU Instant Hot Water Heater #ad
★★★★ 4.4 · 618+ · $254
Amazon
A. O. Smith Tankless Water Heater Natural Gas - Endless Instant Hot Water up to 10 GPM - Integrated Recirculation Pump - Indoor Condensing 199,000 BTU ProLine® XE ATI-540P-N #ad
★★★★☆ 4.9 · · $1160
Amazon
A. O. Smith Tankless Water Heater - Natural Gas - 160,000 BTU - Endless Hot Water - Ultra-Low NOx - X3™ Scale Prevention - ProLine® XE ATI-240HX3-N #ad
★★★★☆ 4.9 · · $1300
Amazon
Electric Tankless Water Heater WINTEMP 27kW 240V, Whole House Hot Water Supply, On Demand Instant Endless Water Heater, with LED Temperature Display, WiFi Control, Up to 6.3 GPM, ETL Certified #ad
★★★★ 4.1 · 251+ · $340
Amazon
A. O. Smith Tankless Water Heater Natural Gas - Endless Instant Hot Water up to 10 GPM - Integrated Recirculation Pump - Indoor Condensing 199,000 BTU ProLine® XE ATI-540P-N #ad
★★★★ 4 · · $1160
Amazon
GASLAND Gasmart E 199,000 BTU Tankless Water Heater, Up to 7.6 GPM, CSA Certified Whole House Natural Gas Indoor Installation Residential On-demand Instant Hot Water Heater, CE686NG #ad
★★★★ 4.2 · 38 · $560
Amazon
Noritz GQ-C3261WX-FF US NG EZ Series Pro 11.1 GPM 199,900 BTU 120 Volt Residential Indoor/Outdoor Natural Gas Tankless Water Heater with Bluetooth Connectivity - Silver #ad
★★★★ 4.1 · 3 · $1799
Amazon
Westinghouse Tankless Water Heater Electric 14kW 240V, Instant On-Demand Hot Water, Smart WiFi, Compatible with Alexa & Google Home, ETL Certified IPX4, Space-Saving, Gray (APOLO-2.0 Upgrade) #ad
★★★★ 4.4 · 90 · $228
Amazon
Tankless Water Heater Natural Gas, 4.21 GPM 16L Indoor Instant Hot Water Heater with Digital Fahrenheit Display Multi-Protection Constant Temperature for Home & Apartment #ad
★★★★ 4.2 · 1 · $114
Amazon
These Amazon-available picks meet the calculated spec. For region-specific or licensed contractor-installed options, consult a local professional. See our Disclaimers.
When to size up or down
- 1–2 person household: 130–150k BTU gas (Bradford White RTGR130, RTGR150).
- 3–4 person household: 180–199k BTU gas (Noritz NCC199CDV, Rheem RTG199).
- 5–6 person household: 250k BTU or two 199k units (Rinnai RU199iN pair).
- Propane vs natural gas: Same BTU sizing. Propane delivers 2,516 BTU/ft3 vs NG 1,030 BTU/ft3 (2.4×volume).
Frequently asked questions
How many BTU for a gas tankless?
Per DOE: BTU = GPM ×500 ×ΔT. For 7 GPM at 65°F rise: 7 ×500 ×65 = 227,500 BTU — round up to 250k BTU. Standard residential sizes: 130k, 150k, 180k, 199k, 250k BTU. Real certified units: Noritz NCC199CDV (199k BTU, 5.5 GPM, UEF 0.98), Bradford White RTGR150 (150k BTU, 4.3 GPM).
Is 199k BTU enough for a family of 4?
Yes, 199k BTU delivers 5.5–8 GPM at 67°F rise — enough for a family of 4 with 2 showers + kitchen + dishwasher. For larger households or cold climates, consider 250k BTU. Top 199k certified: Noritz NCC199CDV (5.5 GPM at 67°F rise, UEF 0.98).
Do I need to upgrade my gas line for tankless?
Often yes. Standard 1/2" gas lines can support up to 199k BTU. For 250k+ BTU tankless, you'll likely need a 3/4" gas line upgrade ($300–$800). Have a licensed plumber assess your gas line before installation.
Natural gas or propane tankless?
Natural gas is cheaper per BTU (typically $1.20/therm vs $2.50/gallon propane — $2.80/therm equivalent). Propane is necessary where natural gas isn't available (rural areas). Both deliver similar performance; propane uses 2.4×the volume (2,516 BTU/ft3 vs NG 1,030 BTU/ft3).