Aisha K.
Mar 12, 2026
Clean layout and the results update instantly. Exactly what I needed for a quick planning check.
Plumbing & Piping
Tankless Water Heater Size Calculator — GPM by Fixtures
Required GPM = sum of simultaneous fixture flow rates.
Sum fixture GPM demands for peak simultaneous hot water use. Planning estimates only — verify measurements and local codes on site.
Inputs
15
04
05
4090
Smart analysis
Recommendations
Tailored to your live inputs and result—guidance only, not professional advice.
- Interpret
Treat this as a planning estimate
Primary result: Required GPM = 5.0. Re-run with optimistic and pessimistic inputs to understand the range—not just a single point.
Scenario studio
Compare A vs B
Capture two sets of inputs on this device, then see which outcome wins. Share either scenario with a link.
Scenario A
Adjust inputs, then capture this slot.
Scenario B
Adjust inputs, then capture this slot.
Plain-English guide
Terms & how it works
Confused by a field? Read the short definitions here while you use the tool.
Shower GPM
The “Shower GPM” value used in this tool’s formula. Adjust it to see results update live.
Kitchen / appliance GPM
The “Kitchen / appliance GPM” value used in this tool’s formula. Adjust it to see results update live.
Other fixtures GPM
The “Other fixtures GPM” value used in this tool’s formula. Adjust it to see results update live.
Temperature rise (°F)
The “Temperature rise (°F)” value used in this tool’s formula. Adjust it to see results update live.
How this calculator works
Sum fixture GPM demands for peak simultaneous hot water use. Planning estimates only — verify measurements and local codes on site. Required GPM = sum of simultaneous fixture flow rates. Example: Shower 2.5 + kitchen 1.5 + lav 0.5 GPM → 4.5 GPM unit needed.
Required GPM = sum of simultaneous fixture flow rates.
Example: Shower 2.5 + kitchen 1.5 + lav 0.5 GPM → 4.5 GPM unit needed.
When to use this Tankless Water Heater GPM
Common use cases
- Sum fixture GPM demands for peak simultaneous hot water use.
- Sanity-check with this case: Shower 2.5 + kitchen 1.5 + lav 0.5 GPM → 4.5 GPM unit needed.
- The relationship is: Required GPM = sum of simultaneous fixture flow rates.
Localized examples
- Plumbing & Piping: the example on this page uses the same formula as the widget.
What you get
- Fields: Shower GPM, Kitchen / appliance GPM, Other fixtures GPM, Temperature rise (°F)
- Required GPM = sum of simultaneous fixture flow rates.
- Example on the page: Shower 2.5 + kitchen 1.5 + lav 0.5 GPM → 4.5 GPM unit needed.
How to Calculate Tankless Water Heater GPM Step-by-Step
- 1Enter Shower GPM, Kitchen / appliance GPM, Other fixtures GPM, then the remaining fields. Use the values you have, not a catalog average, unless that is all you know.
- 2Required GPM = sum of simultaneous fixture flow rates.
- 3Worked case: Shower 2.5 + kitchen 1.5 + lav 0.5 GPM → 4.5 GPM unit needed.
- 4Buy materials after you walk the job. This is a first-pass quantity, not a purchase order.
Governing formula
Required GPM = sum of simultaneous fixture flow rates.
Shower 2.5 + kitchen 1.5 + lav 0.5 GPM → 4.5 GPM unit needed.
Example: set Shower GPM = 2.5, Kitchen / appliance GPM = 1.5, Other fixtures GPM = 1, then read the result.
Variable definitions
- showerGpm
Shower GPM
The “Shower GPM” field. Default 2.5; typical range 1–5.
- kitchenGpm
Kitchen / appliance GPM
The “Kitchen / appliance GPM” field. Default 1.5; typical range 0–4.
- otherGpm
Other fixtures GPM
The “Other fixtures GPM” field. Default 1; typical range 0–5.
- tempRiseF
Temperature rise (°F)
The “Temperature rise (°F)” field. Default 60; typical range 40–90.
- The takeoff (Required GPM = sum of simultaneous fixture flow rates.) ignores waste, code, and what you find when you open the wall. Measure twice on site.
- Nothing here is uploaded for the calculation itself.
Frequently Asked Questions
Required GPM = sum of simultaneous fixture flow rates.
Related calculators
Plumbing & Piping
- Water Heater Sizing CalculatorEstimate recommended tank water heater capacity from household size and peak hot-water demand.
- Water Pressure Loss CalculatorApproximate friction pressure loss in water pipe runs.
- Septic Tank Size CalculatorSize septic tank from bedroom count and daily wastewater flow.
- Bathroom Rough-In Cost CalculatorBallpark rough-in cost from fixtures and labor hours per fixture.
- Drain Slope CalculatorCompute drain grade from horizontal run and vertical drop.
- Fixture Unit Load CalculatorEstimate total DWV fixture units for drain sizing.
Get a note when we add a planner
Occasional email when something new is worth opening. We never share your address. Unsubscribe anytime.
Discover & search
Explore More Tools
Related tools in Plumbing & Piping — strengthening topical internal links.
Same category · 3 tools
Browse all →Water Heater Sizing Calculator
Plumbing & Piping
Estimate recommended tank water heater capacity from household size and peak hot-water demand.
Pipe Length & Fittings Calculator
Plumbing & Piping
Total pipe = straight run + fittings allowance per joint.
Drain Slope Calculator
Plumbing & Piping
Compute drain grade from horizontal run and vertical drop.
Community
Reviews & Ratings
See what others think about this tool, then leave your own rating to help improve CalculioHub.
4.8
out of 5.0
4 reviews
Daniel R.
Feb 28, 2026
Very usable on mobile. Would love a save/export option later, but the math feels solid.
Priya S.
Feb 3, 2026
The FAQ section answered my questions before I even had to search. Smooth dark mode too.
Marcus L.
Jan 19, 2026
Simple inputs, clear outputs. This replaced three bookmarks I used to juggle.
Improve this tool
Suggestion Box
Share an idea, report a bug, or send general feedback for Tankless Water Heater GPM Calculator. Your input helps shape what we build next.