SCS Curve Number & Rainfall Into Runoff
Estimates runoff
“Real reviews from real users decide this tool's public rating.
Enter rainfall and a curve number (CN) for your soil and land use to estimate runoff and infiltration, plus the potential retention S and initial abstraction Ia — to size ponds, bunds and waterways.
SCS curve number runoff
Next: size your waterways and farm ponds for ~50.5 mm of runoff per 100 mm storm; raising soil cover and infiltration lowers the effective CN and keeps more of the 49.5 mm in the profile.
CN bundles soil hydrologic group, land use and antecedent moisture. Runoff is zero until rainfall exceeds the initial abstraction Ia = 0.2·S.
Curve number runoff — key facts
- Retention S
- (25400 ÷ CN) − 254 mm
- Initial abstraction Ia
- 0.2 × S
- Runoff Q
- (P − Ia)² ÷ (P − Ia + S)
- Infiltration
- P − Q
- High CN
- Compacted, bare, clay → more runoff
- Low CN
- Sandy, vegetated → more infiltration
- CN range
- ≈ 30–100
- Privacy
- Runs in your browser; nothing uploaded
One number decides how much rain runs off
When rain falls, some soaks into the ground and some runs off the surface. The SCS curve number method captures that split with a single index — the curve number — that bundles soil type, land use and how wet the ground already is. A high CN (compacted, bare or clay ground) sheds most of the rain as runoff; a low CN (sandy, well-vegetated soil) lets most of it infiltrate. From the CN the method derives the potential retention S, the initial abstraction Ia, and finally the runoff depth.
This tool computes potential retention S, initial abstraction Ia, runoff and infiltration for your rainfall and curve number. Use it to size farm ponds, contour bunds, percolation tanks and grassed waterways, and to see how better land management lowers runoff and stores more water in the soil. Pair it with the Effective Rainfall, Rainwater Harvesting and Farm Pond tools to plan a complete water-harvesting system.
Split rain into runoff and soak
See exactly how much each event yields.
Capture the curve number
Soil, land use and wetness in one index.
Size your structures
Runoff volume sizes ponds, bunds and dams.
Reward good management
Lower CN means more water in the soil.
Frequently Asked Questions
What is the SCS curve number method?+
The SCS-CN method (from the US Soil Conservation Service, now NRCS) estimates how much of a rainfall event becomes surface runoff and how much infiltrates. It packs soil type, land use and antecedent wetness into a single number — the curve number (CN) from 0 to 100 — making it one of the most widely used runoff tools in agriculture and watershed design.
How is runoff calculated from the curve number?+
First the potential maximum retention S = (25400 ÷ CN) − 254 in mm. The initial abstraction Ia (rain absorbed before runoff starts) is taken as 0.2 × S. Then runoff Q = (P − Ia)² ÷ (P − Ia + S) when rainfall P exceeds Ia, otherwise zero. Infiltration is simply P − Q.
What is the curve number (CN)?+
CN is a dimensionless index from about 30 to 100 that summarises how readily a surface produces runoff. Low CN (sandy, well-vegetated, undisturbed) means most rain soaks in; high CN (compacted, bare, clay, paved) means most rain runs off. Standard tables give CN by hydrologic soil group and land use/cover.
What is potential retention S?+
S is the maximum depth of water the soil and surface could absorb after runoff begins, in mm. It's derived directly from CN: a high CN gives a small S (little can be absorbed, lots runs off) and a low CN gives a large S. S is the heart of the method — everything else follows from it.
What is initial abstraction Ia?+
Ia is the rainfall that is intercepted, ponded in surface depressions or infiltrated before any runoff appears. The standard SCS assumption is Ia = 0.2 × S. Rainfall must exceed Ia before runoff starts, which is why a light shower on dry, absorbent ground produces no runoff at all.
What does CN tell me about my soil and land use?+
A higher CN flags more runoff risk: bare or compacted fields, heavy clay, and built or paved areas all push CN up. Lower CN comes from cover crops, mulch, contour cultivation, deep sandy soils and good structure. Lowering CN through management means more water stored in the soil and less erosive runoff.
How is this used to size farm ponds and bunds?+
Runoff depth × catchment area gives the runoff volume from a design storm — the basis for sizing farm ponds, percolation tanks, contour bunds, check dams and grassed waterways. Get the runoff right and you build structures that capture or safely convey the water without being over- or under-sized.
How does antecedent moisture affect runoff?+
Wet ground runs off far more than dry ground. The method handles this with antecedent moisture conditions (AMC I dry, II average, III wet); CN is adjusted upward for wet conditions. Designing for a wet catchment (AMC III) is common for safety, since the biggest runoff events often follow earlier rain.
What are the limits of the CN method?+
It's an event-based estimate, best for design rather than precise prediction, and it's sensitive to the chosen CN and the Ia assumption. It doesn't model rainfall intensity within the storm or detailed soil physics. Used with sensible curve numbers it remains an excellent, practical tool for farm-scale water harvesting and drainage.
Related farming tools
1900+ free tools, zero sign-up
Calculators, developer & SEO utilities, data, content, hardware and farming — all instant, private and in your browser.
Countdown Timer
Open FarmingCold Storage Energy Cost Calculator
Free cold storage energy cost calculator. Enter the cooling load, COP, run hours and tariff to get the input p
Open Dev & SEOWhat Does This npm Package Really Install?
Type any npm package or paste your package.json and resolve the full transitive dependency tree live from the
Open NicheSEO ROI Calculator
Diamond-grade SEO ROI calculator with a 24-month traffic sigmoid forecast (S-shaped ramp curve), red ranking-d
Open ContentImage Color Extractor
Extract a ranked palette of dominant and accent colors from any image, logo or screenshot, with coverage perce
Open HardwareCPU & GPU Tier List Pro
The hardware hierarchy measured five ways: 130+ CPUs & GPUs (2015–2025) ranked into live S–D tiers by performa
Open ProductivityPassword Tools
Test password strength against real standards (NIST 800-63B, OWASP ASVS), check against known breaches via k-a
Open ProRead the Full Text
Read the Vigyan Bhairav Tantra (Vijñāna Bhairava) verse by verse — Sanskrit (Devanagari), IAST transliteration
Open CalculatorCron Parser
Open FarmingCold Storage Shelf-Life Calculator
Free cold-storage shelf-life calculator. Pick a commodity and storage temperature & humidity to see how many d
Open Dev & SEORegex Tester & Builder
Free online regex tester with real-time match highlighting, capture-group inspection, replace/substitution, a
Open NicheSocial Media ROI Calculator
Diamond-grade social media ROI calculator with a 6-axis engagement radar (reach, impressions, engagement-rate,
Open ContentImage Color Picker
Pick exact pixel colors from any image with a precise eyedropper and live magnifier. Drag pins, build a palett
Open HardwareSilicon Architecture Explorer
Explore CPU and GPU silicon: group every part by microarchitecture (Zen, Ada Lovelace, RDNA, Ampere, Blackwell
Open ProductivityPassword Strength Tester
Open ProSettings
Choose your theme, reading size, and script (Devanagari / IAST), and export, import, or clear your private pra
Open CalculatorCron Generator
Open FarmingControlled / Modified-Atmosphere Storage Gas Calculator
Free controlled-atmosphere (CA/MA) storage gas calculator. Pick a fruit or vegetable to get its recommended O2
Open Dev & SEOUUID Generator
Free online UUID generator and toolkit. Generate UUID v4, v7 (time-ordered), v1, v3, v5, v6 and v8 — single or
Open NicheTech ROI Calculator
Diamond-grade technology ROI calculator with a vertical savings ladder by cost category (licensing, infra, int
Open ContentMulti-Format Color Converter
Convert any color between HEX, RGB, HSL, CMYK, LAB, LCH and OKLCH at once — live, with WCAG contrast, relative
Open HardwareCPU GPU Compare Pro
Compare any two CPUs or GPUs side by side: metric-by-metric table with the winner highlighted, the performance
Open ProInstagram Reel to SRT
Paste an Instagram Reel link and get accurate SRT subtitles in seconds. Free, no sign-up, runs in your browser
Open CalculatorCST to EST Converter
Convert CST to EST instantly. Central Time is 1 hour behind Eastern Time year-round (both observe DST). Two-ci
Open