Washington Irrigation Calculator: How Much Water?

The Washington irrigation calculator on this page works out the volume of water needed to irrigate a rectangular lawn or garden bed, based on the area you enter and the depth of water you want to apply. You input the length and width of the area in feet (or meters) and the desired irrigation depth in inches (or centimeters), and the calculator returns the water volume in cubic yards and cubic feet. This is the same math used by landscapers and irrigation designers to size watering schedules, estimate water bills, and plan drip or sprinkler systems.

Formula

The calculation is straightforward volume: Volume = Length × Width × Depth. The only tricky part is converting between units, because the inputs are in feet and inches while the output is in cubic yards and cubic feet.

• L = lawn length in feet (ft)
• W = lawn width in feet (ft)
• D = irrigation depth in inches (in)
• Volume in cubic feet = L × W × (D / 12)
• Volume in cubic yards = L × W × (D / 12) / 27, which simplifies to L × W × D / 324

The factor 12 converts inches to feet, and 27 converts cubic feet to cubic yards (since 1 yard = 3 feet, and 3³ = 27).

Worked Example

Using the default values from the calculator: length = 40 ft, width = 25 ft, depth = 3 inches.

1. Convert depth to feet: 3 in ÷ 12 = 0.25 ft
2. Calculate area: 40 ft × 25 ft = 1,000 ft²
3. Calculate volume in cubic feet: 1,000 ft² × 0.25 ft = 250 ft³
4. Convert to cubic yards: 250 ft³ ÷ 27 = 9.26 yd³ (rounded to 2 decimals)

The calculator shows 250 ft³ and 9.26 yd³, matching the manual calculation.

Why Washington-Specific?

Washington State has a wide range of climates, from the wet western side (Seattle gets about 37 inches of rain per year) to the arid eastern side (Yakima gets about 8 inches). The state’s Department of Ecology and local water utilities often publish regional irrigation guidelines, but the underlying math is universal. This calculator helps you apply the right amount of water regardless of where you live, by letting you set the depth you want to apply. For example, a common recommendation for lawns in the Pacific Northwest is 1 to 1.5 inches of water per week during the dry summer months, but that varies with soil type, grass species, and temperature.

How to Use the Result

The volume tells you how much water you need to deliver to the area. To translate that into watering time, you need to know your irrigation system’s flow rate. For instance, a typical oscillating sprinkler delivers about 1 inch per hour over its coverage area. If you want to apply 1 inch of water to a 1,000 ft² lawn, you need 1,000 ft² × (1/12) ft = 83.3 ft³ of water, which is about 623 gallons (since 1 ft³ = 7.48 gallons). If your sprinkler puts out 1 inch per hour, you’d run it for 1 hour. If your drip system delivers 0.5 inches per hour, you’d run it for 2 hours. The calculator gives you the volume; you combine it with your system’s precipitation rate to get run time.

Common Mistakes and Misconceptions

• Mixing units: Entering length in yards but width in feet will give a wrong area. Always use the same unit for length and width.
• Forgetting to convert depth: Depth is in inches, but volume formulas require feet. The calculator handles this, but if you do it by hand, don’t forget to divide by 12.
• Assuming 1 inch of water = 1 gallon per square foot: Actually, 1 inch of water over 1 ft² is 0.623 gallons (since 1 ft³ = 7.48 gallons, and 1/12 of a cubic foot is 7.48/12 ≈ 0.623).
• Overwatering in spring/fall: Washington’s cooler months often require less irrigation than summer. Use a rain gauge or soil moisture sensor to adjust.

Alternatives and Related Calculations

If you’re planning a new lawn, you might also need to estimate the amount of sod or seed. Sod is usually sold by the pallet, and coverage varies by supplier, so you’d enter the pallet coverage as an input. For seed, the rate is typically 5-10 lbs per 1,000 ft², but again, check the label. This calculator only handles water volume; for soil amendments or fertilizer, you’d use a different calculation based on the product’s recommended rate.

Limitations and Accuracy

The calculator assumes a flat, rectangular area. If your lawn has curves or slopes, break it into smaller rectangles and add the volumes. It also doesn’t account for water lost to runoff, evaporation, or deep percolation. In practice, you may need to apply 10-20% more water than the theoretical volume to account for inefficiencies, especially with sprinklers on a windy day. The result is a good starting point, but local conditions and your irrigation system’s uniformity matter.

Frequently Asked Questions

How many gallons is that volume?

To convert cubic feet to gallons, multiply by 7.48. For the default example (250 ft³), that’s 1,870 gallons. This is a quick way to estimate water usage for billing or well capacity.

What depth should I use for my lawn?

Most turf grasses need about 1 to 1.5 inches of water per week during active growth, but it’s better to water deeply and infrequently (e.g., 1 inch twice a week) than to water shallowly every day. In Washington’s cooler coastal areas, 1 inch per week is often enough; in the eastern part of the state, you may need 1.5-2 inches during heat waves.

Can I use this for a garden bed?

Yes, the math is the same. Vegetables and flowers typically need about 1 inch of water per week, but container plants may need more frequent watering because they dry out faster. Adjust the depth based on your soil and plant needs.

How do I know my sprinkler’s flow rate?

Place several straight-sided cans (like tuna cans) in the watering area, run the sprinkler for 15 minutes, then measure the depth of water in each can. Average the depths and multiply by 4 to get inches per hour. That’s your precipitation rate.

Does Washington have water restrictions?

Some municipalities in Washington have watering restrictions during summer, especially in the eastern part of the state. Check with your local water utility or the Department of Ecology for current rules. This calculator doesn’t provide legal advice; it just helps you apply the right amount.

What if my lawn is not a perfect rectangle?

Divide the area into smaller rectangles or triangles, calculate each with the same formula, and sum the volumes. For irregular shapes, you can also use a measuring wheel and approximate the area, but the more sub-areas you use, the more accurate the result.

Related

• Irrigation Calculation Formula & How to Use It
• Irrigation Calculation Sheet: How to Build One
• Hunter Sprinkler Zone Calculator: How to Plan Zones

in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1