Size Tankless Water Heater Calculator
Sizing a tankless water heater is different from sizing a tank unit: you size by peak simultaneous flow (GPM) and required temperature rise, not by stored gallons. This size tankless water heater calculator combines household simultaneous-fixture demand with your local inlet water temperature to return the exact GPM and BTU input your unit needs, and checks whether a gas or electric tankless can meet the spec.
How to Size a Tankless Water Heater
Tankless sizing is two variables: GPM (gallons per minute) and temperature rise.
GPM: Simultaneous Flow Demand
Add up the flow rates of fixtures you might run at the same time:
| Fixture | Flow Rate |
|---|---|
| Low-flow shower | 2.0 GPM |
| Standard shower | 2.5 GPM |
| Kitchen sink | 1.5 GPM |
| Bathroom sink | 1.0 GPM |
| Dishwasher | 1.5 GPM |
| Clothes washer | 2.0 GPM |
Example: 2 showers + kitchen sink = 2(2.0) + 1.5 = 5.5 GPM
Temperature Rise
Temperature rise = desired output (120F standard) minus inlet water temperature. Inlet water temp varies by climate:
| Region | Inlet Temp | Rise to 120F |
|---|---|---|
| Warm (FL, TX, AZ) | 70F | 50F |
| Moderate (CA, Mid-Atlantic) | 55F | 65F |
| Cold (Midwest, NE) | 45F | 75F |
| Very cold (Northern Plains) | 35F | 85F |
BTU Input Required
BTU/hr = GPM x Temp Rise x 500
Example: 5.5 GPM x 65F rise x 500 = 178,750 BTU/hr. That needs a premium residential gas tankless like a Rinnai RU180 or RU199, or a Navien NPE-240A2.
Tankless Water Heater Size Chart
The tankless water heater size chart below maps household size to GPM and unit class:
| Household | Bathrooms | GPM Needed | Typical Unit |
|---|---|---|---|
| 1-2 people | 1 bath | 3-5 GPM | Rinnai V65, Rheem RTG-64 |
| Family of 4 | 2 baths | 7-8 GPM | Rinnai V75, Rheem RTGH-84 |
| Family of 5-6 | 3 baths | 9-10 GPM | Rinnai RUC98, Navien NPE-240A2 |
| 6+ people | 4+ baths | 10-12 GPM | Cascaded or commercial unit |
Tankless Water Heater Size for Family of 4
Tankless water heater size for family of 4 with 2 bathrooms is typically 7-8 GPM in moderate climates. In cold climates, the same family needs a 9-10 GPM unit because the temperature rise is higher. A 7 gpm tankless water heater covers most families of 4 in warm-to-moderate climates; step up to 10 gpm tankless water heater in cold climates.
10 GPM Tankless Water Heater
A 10 gpm tankless water heater handles 3-4 simultaneous fixtures in warm climates, 2-3 simultaneous fixtures in cold climates. Popular 10 GPM units: Rinnai RUC98i (199k BTU gas), Navien NPE-240A2 (199k BTU gas), Stiebel Eltron Tempra 36 (36 kW electric, 4 circuits).
10 GPM Tankless Water Heater Electric
A 10 gpm tankless water heater electric is only practical in warm climates (70F inlet). Units like the Stiebel Eltron Tempra 36 or EcoSmart ECO 36 deliver 10 GPM at a 35-40F temperature rise (warm climates), but drop to 6-7 GPM at the 65-75F rise cold climates require. For 10 GPM in cold climates, you need gas.
7 GPM Tankless Water Heater
A 7 gpm tankless water heater suits a family of 4 with 2 bathrooms in warm-to-moderate climates (50-65F rise). Gas models: Rinnai V75iN (180k BTU), Takagi T-K4 (190k BTU). Electric: Stiebel Eltron Tempra 29 (29 kW, 3 x 40A circuits).
7 GPM Tankless Water Heater Electric
A 7 gpm tankless water heater electric is borderline: the Tempra 29 or EcoSmart ECO 29 delivers 7 GPM at 50F rise (warm climate) but drops to 4-5 GPM at 75F rise (cold). Confirm your electrical panel can support 3 x 40A circuits (120A of dedicated load, 200A service minimum).
5 GPM Tankless Water Heater
A 5 gpm tankless water heater covers 1-bathroom homes or couples with 2 bathrooms. In gas: Rheem RTG-64 (140k BTU), Rinnai V53. In electric: Stiebel Eltron Tempra 24 (24 kW, 2 x 40A circuits). Works in all climates when sized correctly.
What Size Electric Tankless Water Heater Do I Need
What size electric tankless water heater do I need depends on climate. The electric tankless water heater size chart:
| Family / Bathrooms | Warm Climate (70F inlet) | Cold Climate (45F inlet) |
|---|---|---|
| 1 person / 1 bath | 10-12 kW (3-4 GPM) | 18-24 kW (3 GPM) |
| Couple / 1 bath | 13-18 kW (4-5 GPM) | 24-29 kW (4 GPM) |
| Family of 4 / 2 bath | 24-29 kW (6-7 GPM) | 29-36 kW (5 GPM) |
| Family of 5 / 3 bath | 29-36 kW (8-10 GPM) | Requires gas |
Electric tankless above 29 kW requires 200A service panel and 3-4 dedicated 40A breakers. In cold climates, gas is almost always better for whole-house tankless.
Rinnai Tankless Water Heater Models
Popular Rinnai tankless water heater models:
- Rinnai V53DeN: 5.3 GPM, 120k BTU, entry level
- Rinnai V65eN: 6.5 GPM, 150k BTU, couple or small family
- Rinnai V75iN: 7.5 GPM, 180k BTU, family of 4 moderate climate
- Rinnai V94eN: 9.4 GPM, 199k BTU, family of 4 cold climate
- Rinnai RUC98iN: 9.8 GPM, 199k BTU, condensing (higher efficiency)
- Rinnai RUCS65iN: 6.5 GPM, 160k BTU, ultra-condensing
Electric Tankless Water Heater Size Chart
An electric tankless water heater size chart keyed to kW rating:
| kW | Max GPM (warm) | Max GPM (moderate) | Max GPM (cold) |
|---|---|---|---|
| 13 kW | 3.5 | 2.5 | 1.8 |
| 18 kW | 5.0 | 3.5 | 2.5 |
| 24 kW | 6.5 | 5.0 | 3.5 |
| 27 kW | 7.5 | 5.5 | 4.0 |
| 29 kW | 8.0 | 6.0 | 4.5 |
| 36 kW | 10.0 | 7.5 | 5.5 |