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How Do I Calculate Compost Application Rates?

By: Jessie Keith

“Your example calculation on your Guide’s formula for Compost application seems to be incorrect. What is right?  I am trying to figure how much to order.” Question from Anne of Elmdale, Kansas 

Answer: The original formula did need some adjustment, so I just modified it. Here is the corrected formula along with a conversion table and links to other helpful sources–most notably, a great article for easily calculating the square footage of beds of various shapes.

Amendment Application Formula for Beds

For square or rectangular beds, multiply the bed’s length by its width to get square footage (L *W= ft2). If you wish to lay mulch 2-inches deep, then have a look at the table below to get the corresponding square footage covered by 1 cubic yard of compost, mulch, or peat moss. Then divide the square footage in the table that matches the 2-inch depth.

For example: If you had a 12-foot x 24-foot bed that required 2 inches of peat moss for tillage, calculate 12 feet x 24 feet = 288 square feet, then from the chart you can determine that 288 ft2/162 ft = 1.78 cubic yards of mulch. 

Square Feet to Cubic Yards Conversion Table

1 cubic yard of an amendment or mulch will cover the following square footage to each depth.

Depth of Amendment

Square Footage Covered

1”

324 ft2

2”

162 ft2

3”

108 ft2

4”

81 ft2

To calculate the square footage of other bed shapes, please click here for an excellent reference. You can also click here to view a handy mulch/amendment calculator.

Spring is the best time to liberally apply compost or mulch to gardens, but if you also plan to do significant fall planting, reapply in autumn. Organic matter breaks down over the course of the season and needs to be replenished. If you plan any winter or cold-frame gardening, apply compost as both an amendment and protective mulch.

Happy gardening,

Jessie Keith

Black Gold Horticulturist

About Jessie Keith


Plants are the lens Jessie views the world through because they’re all-sustaining. (“They feed, clothe, house and heal us. They produce the air we breathe and even make us smell pretty.”) She’s a garden writer and photographer with degrees in both horticulture and plant biology from Purdue and Michigan State Universities. Her degrees were bolstered by internships at Longwood Gardens and the American Horticultural Society. She has since worked for many horticultural institutions and companies and now manages communications for Sun Gro Horticulture, the parent company of Black Gold. Her joy is sharing all things green and lovely with her two daughters.

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