About Irrigation Calculators
Free irrigation and watering calculators built on established engineering methods, with the formulas, assumptions and sources on every page, so you can trust the answer and check it yourself.
Why this site exists
Watering decisions are usually made with rules of thumb: "an inch a week", "twenty minutes a zone", "one-inch pipe should be fine". Sometimes those rules work. Often they waste water, starve plants, or lead to systems that never perform the way they should.
The methods that give better answers already exist. They're in FAO and USDA handbooks, Irrigation Association practice, ASABE standards and university extension publications. But they're spread across technical documents, written for specialists, and rarely turned into tools people can use in a minute on a phone.
Irrigation Calculators turns those methods into clear, fast calculators. Each one answers a specific question, shows how it got the answer, and is honest about what it can't know about your site. Our aim is simple: help homeowners, landscapers and growers make better water-use decisions based on real engineering methods rather than rules of thumb.
Who it's for
Homeowners
Find out how much and how long to water your lawn and garden, how to set your sprinkler timer, and what your watering really costs.
Landscape and irrigation contractors
Check precipitation rates, zone flows, pipe sizes and run times on site, and give clients numbers you can explain.
Irrigation designers and auditors
Quick, transparent checks for uniformity, hydraulics, scheduling and water budgets, with the method and source stated.
Farm managers and growers
Crop water requirements, system capacity, set times, pivots, drip tape and fertigation, in acre-inches, gallons or metric units.
What you'll find here
Our calculators are organized into ten categories, from a single lawn to a center pivot:
- LawnHow much and how long to water, schedules, new seed and sod
- SprinklersPrecipitation rates, run times, spacing, zones and audits
- Drip irrigationEmitters, run times, zone flow, tubing and filters
- Irrigation hydraulicsPipe sizing, friction loss, pressure and supply flow
- Scheduling and ETEvapotranspiration, water requirements and water budgets
- Soil and water qualityWater holding capacity, infiltration and salinity
- AgricultureCrop water needs, center pivots, set times and well capacity
- Garden and treesBeds, containers, trees, shrubs and rainwater
- FertigationInjection rates, injector ratios and fertilizer ppm
- Cost and savingsWater costs, system costs and upgrade savings
Alongside the calculators, our reference charts collect the underlying data in one place: Sprinkler precipitation rate chart, crop coefficient table, soil infiltration rate chart, pvc friction loss chart, lawn watering by grass type. Browse all calculators or all reference charts.
What makes our calculators different
Plenty of websites will give you a number. We want you to be able to trust it, and to see exactly where it came from.
Formulas on the page
Every calculator shows the formula it uses and a worked example, so you can check the math yourself.
Named sources
Methods come from FAO, USDA NRCS, the Irrigation Association, ASABE and university extension services, and each page lists its sources.
Assumptions stated
Each result comes with the assumptions behind it, and they're included when you copy, download or print it.
Honest precision
Estimates are rounded to what the inputs can support and labeled as estimates. Exact conversions stay exact.
US and metric
Switch units at any time. The values you've entered convert automatically and the result stays the same.
Built for the field
Every calculator works on a phone, updates as you type, and gives you a link that reopens it with your values.
Free, with no sign-up
No accounts, no email gates and no paywalls. Use the tools as often as you like.
Private by design
Calculations run in your browser. The values you enter aren't sent to us.
Who's behind the calculators
Daniel Reeves, PE, CID
Irrigation Design Engineer & Water-Efficiency Specialist
Daniel is a licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years of experience in irrigation system design, performance auditing and landscape water management. He spent a decade designing systems for parks, golf courses, HOAs and large residential properties, then carried out hundreds of irrigation audits as a water-efficiency specialist for a municipal water agency.
On this site, Daniel develops, validates and reviews the calculators and reference data. Read his full profile.
How we build and check every calculator
- Start from an established method. Every formula comes from a recognized standard, handbook or research-based publication. We don't invent formulas or rules of thumb.
- Give each calculator one clear job. Each tool answers one specific question and points you to the right tool for related questions.
- Test it thoroughly. Worked examples are checked by hand against the calculator, and typical, extreme and invalid inputs are tested in both US and metric units.
- Check every source. Every sourced number is checked against the source it's attributed to.
- Engineering review. Daniel reviews the method, formulas and values before a page is published.
- Keep it current. Pages show when they were last reviewed, and we recheck them when sources change or readers report problems.
The details, including the constants, rounding rules and methods by topic, are in our methodology. How we write, review and correct content is in our editorial policy.
The sources we rely on
Our calculators and charts are built on published, authoritative work, including:
- International and government: FAO Irrigation and Drainage Papers, and USDA Natural Resources Conservation Service handbooks and practice standards.
- Professional and standards bodies: Irrigation Association practice and ASABE standards.
- University extension: including University of California, Colorado State, Arizona, Rutgers, Clemson, Iowa State and University of Maryland extension services.
- Manufacturers' published data: used only for equipment-specific values, and always named.
Independent and free
Irrigation Calculators is free to use, with no accounts or paywalls. We don't sell irrigation products or design services, and we don't accept payment to include, rank or recommend products, companies or links. When we name a manufacturer, it's because we're citing its published data.
The site may show advertising to cover its costs. Ads are kept separate from our content and never influence our calculators, results or recommendations.
What our calculators can and can't do
Our results are planning estimates based on the values you enter and published methods. They're a strong starting point for watering, scheduling and sizing decisions, but they don't replace a site-specific design for large or commercial systems, your local watering rules, or the label on a fertilizer or chemical product. Each calculator states its assumptions and tells you when a different tool, or a professional, is the better choice.
Get in touch
We'd like to hear from you, whether you've found an error, want a calculator we don't have yet, or have a question about a result. Use our contact page, and if it's about a calculator, include the page and the values you entered.
You can also read our privacy policy and terms of service.