Our principles
- Established methods only. Every formula comes from a recognized standard, handbook or research-based publication. We don't invent formulas, coefficients or rules of thumb.
- Show the work. Each calculator page shows its formulas, a worked example and its sources, so you can check any result yourself.
- State the assumptions. Every result is shown with the assumptions behind it, and those assumptions are included when you copy, download or print a result.
- Honest precision. Results are rounded to a level the inputs can support, and estimates are labeled as estimates.
- Nothing invented. No made-up statistics, studies, ratings or testimonials. If we can't support a statement with a source or a calculation, we leave it out.
Where our methods and data come from
We use sources in this order of preference:
- International standards, government and professional bodies: for example FAO Irrigation and Drainage Papers, USDA Natural Resources Conservation Service handbooks and practice standards, ASABE standards and Irrigation Association practice.
- University extension and peer-reviewed research: for example publications from University of California, Colorado State, Rutgers, Clemson and Arizona extension services.
- Manufacturers' published data: used only for values that belong to specific equipment, such as a nozzle's precipitation rate or a pipe's friction loss tables. We name the manufacturer whenever we use their data.
We don't use forums, unsourced websites or other calculators as sources. When sources disagree, we say so on the page rather than quietly picking one. When a source contains an evident error, we flag it instead of silently correcting it.
Every source is listed at the bottom of the page that uses it. Where a claim relies on a specific source, we name that source in the text. Numbers attributed to a source are numbers that appear in that source.
Methods by topic
| Topic | Method | Main source |
|---|---|---|
| Reference evapotranspiration | FAO Penman-Monteith; Hargreaves where only temperature data exist | FAO Irrigation and Drainage Paper 56 |
| Crop water use | ETc = Kc × ETo, with stage-based Kc and climate adjustment | FAO-56, Tables 11 and 12 |
| Turf water use | ETo × turf crop coefficient | University of California Cooperative Extension; FAO-56 |
| Sprinkler precipitation rate | 96.25 × GPM ÷ area (square, triangular and whole-zone forms) | Standard irrigation design practice; manufacturers' design guides |
| Pipe friction loss | Hazen-Williams, with a C value for the pipe material (150 for PVC) | Standard hydraulics; pipe manufacturers' engineering data |
| Pipe velocity limit | At or below 5 ft/s at design flow | USDA NRCS Conservation Practice Standard 430 |
| Soil intake and runoff | Infiltration rates by texture; maximum application rates by soil and slope | USDA NRCS; FAO; university extension |
| Water quality and salinity | Salinity, sodium and leaching guidelines | FAO Irrigation and Drainage Paper 29 |
Our reference charts collect the underlying data in one place: crop coefficients, sprinkler precipitation rates, soil infiltration rates, PVC friction loss and lawn watering by grass type.
Units and constants
Every calculator works in US customary and metric units. When you switch units, the values you've entered are converted and the result stays the same physical quantity. We use exact or standard conversion constants:
| Quantity | Value |
|---|---|
| Length | 1 in = 25.4 mm (exact) |
| US gallon | 231 in³ (exact) ≈ 3.785 L |
| Depth to volume (US) | 1 in over 1 sq ft = 144 ÷ 231 ≈ 0.623 gal |
| Depth to volume (metric) | 1 mm over 1 m² = 1 L (exact) |
| Acre and acre-inch | 1 acre = 43,560 sq ft; 1 acre-inch ≈ 27,154 gal |
| Cubic foot | 1 ft³ ≈ 7.48 gal |
| Precipitation rate constant | 96.25 = 231 ÷ 144 × 60 (GPM over sq ft to in/hr) |
| Pressure and head | 1 ft of water ≈ 0.433 psi; 1 psi ≈ 2.31 ft of water |
Rounding and precision
Calculations run at full precision; only the displayed result is rounded. How far we round depends on what kind of result it is:
- Estimates built on targets, weather data or typical values are rounded to honest precision and shown with "about". For example, a weekly watering volume is rounded to the nearest 10 gallons for a typical lawn, because its inputs aren't more precise than that.
- Exact conversions, such as inches of water to gallons, aren't rounded beyond normal display precision and aren't labeled "about".
- Displayed values match the calculation. Depths and rates show 2 decimals in US units and 1 in metric, times show whole minutes, and the rate or depth shown next to a result is the one actually used.
- Your inputs are shown exactly as you entered them in results, copies and downloads.
Assumptions, typical values and verdicts
- Assumptions are listed with every result. For example, a watering calculator states whether it counts rainfall in full or allows for uneven sprinkler coverage.
- Every number is sourced, calculated or labeled "typical". Typical values, such as common sprinkler rates, come with advice to measure your own or check the manufacturer's data.
- Verdicts are calculated, never guessed. A calculator only calls a result low, high, safe or too fast when it compares a calculated value with a sourced threshold, and it shows both numbers.
- Comparisons are like for like. When a page compares two methods, both use the same inputs.
- Requirements and recommendations are kept apart. We say whether a source requires something or only recommends it, and what it applies to.
How every page is checked
Every page follows written standards for sources, formulas and accuracy, and every calculator goes through these checks before it's published:
- Formula check. Each formula is compared with its source, and the formula shown on the page is the one the code uses.
- Worked examples. Each example is worked by hand and must match the calculator's output exactly, including rounding.
- Test cases. Typical, very small, very large and edge-case inputs are run in both US and metric units. Switching units must give the same physical answer.
- Error handling. Empty, zero, negative, non-numeric and unrealistic inputs produce a clear message. No input combination may display an invalid number.
- Source check. Every sourced number and recommendation is checked against the source it's attributed to.
- Engineering review. Daniel Reeves, PE, CID reviews the method, formulas and sourced values.
- Usability check. Pages are tested on phone, tablet and desktop screens and with keyboard-only navigation.
Each page shows the date it was last reviewed and a short list of the checks carried out for that page.
Updates and corrections
We review pages when a source is updated, when we find a better source, or when a reader reports a problem. If you think a result, formula or source is wrong, please tell us and include the page and the values you entered. We check every report. When a correction changes a formula or a result, we update the page and its review date.
Our editorial policy covers how content is written, reviewed and kept independent.
What our calculators are not
- Not a site-specific design. Results are planning estimates. Large, commercial or agricultural systems should be designed by a qualified irrigation designer or engineer who can assess the site.
- Not a substitute for local rules. Watering restrictions, backflow requirements and landscape water ordinances set by your water utility or local authority come first.
- Not a substitute for product labels. For fertilizers and chemicals, the product label and local regulations always take priority over any calculated amount.
The calculators run in your browser. The values you enter are used to calculate your result on your device; share links include your inputs in the web address so the calculator can reopen with them. See our privacy policy for how the site handles data.