Calculate simultaneous hot-water flow and heating input from fixture groups, then compare an entered candidate heater at the same temperature rise.
Trades & Professional Tools · Plumbing
Enter your project measurements
Measured inputs · visitor-entered product and design assumptions
1EnterProvide the known values
2CalculateResults update automatically
3VerifyReview the details and units
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Measure using one consistent system. Product coverage, density, resistance, waste, and safety assumptions must come from the actual product data or project specification. Values stay in this browser.
Trade calculator result
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Build simultaneous hot-water flow from fixture groups and translate temperature rise into heat demand.
At a glance
See the relationship
Cold-water temperatureTarget hot-water temperatureFixture group 1 countFlow per fixture in group 1
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Tankless Water Heater Sizing resultThe calculator connects the entered project values above to the result below. Change one input at a time to see which assumption drives the outcome.
The relationship
The formula used here
Thermal output = ρcpQΔT; required input divides by entered efficiency. Candidate flow and input margins compare product data only at the entered design temperature rise.
This is the relationship applied to your inputs. Read the outputs together rather than treating one displayed number as a complete plumbing decision.
Field reading
Use the result as a next check
Enter real project values
Use site measurements, manufacturer information, and the same unit basis throughout.
Read the full result
Compare a candidate only at the same flow and temperature rise stated in its product data.
Keep the boundary clear
It does not select equipment, combustion air, venting, electrical supply, or a safe installation.
Quick guide
How to use this calculator
Choose the calculation mode or measurement system that matches the values you actually have.
Enter measured project dimensions and use product-specific coverage, density, yield, resistance, or waste data where requested.
Review the intermediate results before ordering material or applying the result on site.
Calculation method
Calculation method
Thermal output = ρcpQΔT; required input divides by entered efficiency. Candidate flow and input margins compare product data only at the entered design temperature rise.
The calculator preserves full internal precision and rounds only displayed results. Quantity outputs round up when whole purchasable pieces are required.
Worked example
Worked example
A 10 L/min simultaneous draw heated by 35 °C needs about 24.4 kW delivered to the water.
Thermal output = ρcpQΔT; required input divides by entered efficiency. Candidate flow and input margins compare product data only at the entered design temperature rise.
Supported inputs
Precision and limits
Water-property assumptions
Where a calculator does not request fluid properties, it models water at 998.2 kg/m³ density and 4,186 J/(kg·K) specific heat. Temperature, pressure, dissolved content, or a different fluid can change real results.
Planning scope
This calculator supports measurement, estimating, and scenario planning. It does not inspect a site, select a product, certify a design, or replace drawings, specifications, manufacturer instructions, local codes, permits, or a qualified trade professional.
Measured and product-specific inputs
Nominal dimensions can differ from actual dimensions. Verify inside pipe diameters, actual lumber sizes, product coverage, bag yield, material density, conductor resistance, waste, and packaging against the materials used on the project.
Safety and regulatory limits
Electrical, HVAC, structural, excavation, roofing, pressure, lifting, and life-safety work may require jurisdiction-specific calculations and professional approval. Do not use a generic planning result as a code-compliance or safety determination.
Precision and privacy
Inputs accept bounded plain decimals. Calculations remain in this browser and raw entered project values are not submitted to 365CALCS.COM.
