Editorial & engineering standards
Our Methodology
How we build, cite, and verify every calculator on DrainageCalculators.com — so engineers, contractors, and educators can trust and reference the results.
Engineering standards we follow
Every calculator implements a documented, published method rather than a black-box formula. The primary standards behind the 51+ tools on this site are:
| Standard | Title | Used for |
|---|---|---|
| FHWA HEC-22 | Urban Drainage Design Manual | Inlet hydraulics, gutter and pavement drainage, storm drain design. |
| NRCS TR-55 | Urban Hydrology for Small Watersheds | Curve number runoff, time of concentration, peak discharge, hydrographs. |
| FHWA HDS-5 | Hydraulic Design of Highway Culverts | Culvert inlet/outlet control sizing and performance. |
| Manning's Equation | Open-channel & pipe flow | Normal depth, velocity, and capacity for channels and gravity pipes. |
| ICC IPC | International Plumbing Code | Roof drainage, gutter and downspout sizing tables. |
| UK Building Regs Part H | Drainage & waste disposal | Foul and surface water drainage falls and gradients (UK). |
| ASCE MOP 77 | Urban Stormwater Management | Catch basin, structure, and BMP design guidance. |
Citation policy
Each calculator lists the specific references behind its method in a Sources panel, formatted in APA style, with the section or equation used. Reference data pages (Manning's n, curve numbers, runoff coefficients, IDF data, pipe dimensions, and more) attribute their source tables to the originating authority (FHWA, NRCS, ASCE, ICC, NOAA).
Verification & testing
Calculation logic is separated from the user interface as pure, testable functions. Those functions are covered by an automated suite of 1,900+ unit tests that check results against worked examples from the source references and guard against regressions. The site is also covered by end-to-end tests and automated link and metadata validation.
Editorial review
Calculations are built from the published standards listed above and verified by the automated test suite. A named licensed Professional Engineer (P.E.) reviews and signs off on calculator methodologies; reviewer details are published on this page and in each calculator's byline.
Limitations
The calculators are intended for preliminary design, estimating, and education. They do not replace a complete design prepared and sealed by the licensed engineer of record, nor local code review. Always confirm results against current standards and the requirements of the authority having jurisdiction.
Frequently asked questions
Where do the formulas and coefficients come from?
Every calculator implements a published engineering method and cites its governing standard — for example FHWA HEC-22 for inlet hydraulics, NRCS TR-55 for curve-number runoff, FHWA HDS-5 for culverts, and the ICC IPC for roof and gutter sizing. Coefficients (Manning's n, runoff coefficients, curve numbers) are drawn from these published sources and listed on the relevant reference pages.
How accurate are the calculators?
Each calculation engine is covered by an automated test suite (1,900+ unit tests) that checks the math against worked examples from the source references. Results are intended for preliminary design and education; final designs should be reviewed and sealed by a licensed engineer responsible for the project.
Are the calculators free?
Yes. Every calculator is free to use with no signup, and most are embeddable on other sites at no cost.
How is the content kept up to date?
Calculators carry a "last verified" date, and methodologies are reviewed when the underlying standards are updated. Corrections can be reported through the feedback control on each calculator.
Found an error or have a source to suggest? Use the feedback control on any calculator, or contact us. We correct verified issues promptly.