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Trades & Professional Tools · Site Work & Landscaping

Rational-Method Peak Runoff Calculator

Calculate a small-catchment peak-flow screen from entered rainfall intensity, area and runoff coefficient.

Trades & Professional Tools · Site Work & Landscaping

Enter your project measurements

Measured inputs · visitor-entered product and design assumptions
  1. 1EnterProvide the known values
  2. 2CalculateResults update automatically
  3. 3VerifyReview the details and units
Try an example

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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Enter valid values to see the result.

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Understand the job

What the Rational-Method Peak Runoff is for

Calculate a small-catchment peak-flow screen from entered rainfall intensity, area and runoff coefficient.

Drainage planning follows contributing area, entered storm basis, runoff assumptions, conveyance, storage, infiltration, overflow, and erosion controls.

Simple visual explanation

Follow the calculation from field input to decision

Define catchment and event
Route flow or storage
Check outlet and field limits
Each stage is deliberately separate: verify the source values, the calculation basis, and the final field or shop check.
Drainage areaEntered design rainfall intensityEntered runoff coefficient (0 to 1)

The relationship

Read the formula before using the result

Q=C×i×A; SI Q(m³/s)=C×i(mm/h)×A(ha)÷360.

The calculator preserves full internal precision. Confirm that every entry uses the same measurement condition, unit basis, product or machine data, and scope represented by this formula.

Professional check

Turn the answer into a responsible next step

1. Verify the inputs

Use actual measurements, current drawings, approved specifications, and manufacturer or process data for the real job.

2. Challenge the assumptions

Check allowances, losses, stock or package rounding, operating condition, and whether the result matches an independent measurement or estimate.

3. Respect the boundary

It does not establish boundaries, legal grades, utilities, excavation safety, drainage approval, irrigation criteria, structural design, chemical application, permits, or environmental compliance.

Quick guide

How to use this calculator

  1. Choose the calculation mode or measurement system that matches the values you actually have.
  2. Enter measured project dimensions and use product-specific coverage, density, yield, resistance, or waste data where requested.
  3. Review the intermediate results before ordering material or applying the result on site.

Calculation method

Calculation method

Q=C×i×A; SI Q(m³/s)=C×i(mm/h)×A(ha)÷360.

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

One hectare at 100 mm/h and coefficient 0.5 produces 0.139 m³/s.

Q=C×i×A; SI Q(m³/s)=C×i(mm/h)×A(ha)÷360.

Supported inputs

Precision and limits

Site, water and landscape-design boundary

These tools use measured site geometry, field observations, product data, approved application rates, locally selected rainfall and runoff inputs, and contractor production assumptions you enter. They do not establish legal grades or boundaries; prescribe fertilizer, irrigation, drainage, planting, soil, pesticide or structural criteria; locate utilities; approve excavation or equipment operation; or replace drawings, permits, local environmental rules and qualified professional review.

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.

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