Rectangular Weir Flow Calculator
Estimate free rectangular-weir discharge from crest width, upstream head and a supplied discharge coefficient.
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Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Integrate ideal velocity over the head above a horizontal rectangular crest, with the entered coefficient.
Q=(2/3) Cd width √(2g) head^(3/2)
Worked example
A 1 m crest, 0.2 m head and Cd=0.62 give approximately 0.163727172 m³/s. Doubling head multiplies flow by 2√2.
Assumptions and input details
Free, fully aerated, uncontracted rectangular nappe with negligible approach velocity. No submergence, end contraction, head measurement correction or standard-specific coefficient selection; not a calibrated gauging result.
Unit definitions
Unit selection is independent of language. Each selector contains only units for its physical quantity. Calculations use defined base units internally.
Choose from 23 length definitions, including explicitly labelled regional measures. Area conversions square the length factor; geometric volumes cube it.
Regional measures are not interchangeable merely because their names look similar. This is a documented selection, not a claim to include every historical unit.
NIST measurement tables · Japanese traditional units · Mainland China and Hong Kong unit distinctions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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