Raised Bed Soil Calculator
Premium garden mix yardage and bag count by bed size
What is raised bed soil calculator?
A raised-bed soil calculator answers the gardener’s first question on any new bed: how many bags or cubic yards do I need to fill it. The volume math is straightforward — length times width times depth — but premium garden mix is lighter and fluffier than plain topsoil, the bag sizes vary at retail, and the first-season settle is significant.
The default density is 50 lb/ft³, which is typical for premium raised-bed mix — a peat, compost, perlite, and topsoil blend engineered to drain well in a confined volume. Heavier blends with more topsoil can run 70-80 lb/ft³ and weigh 50% more per yard. The volume math doesn’t change; only the weight readout differs.
What depth to choose? Twelve inches is the workhorse depth for most vegetable beds — tomatoes, peppers, beans, squash, and the like. Six inches is the minimum and works for shallow-rooted greens (lettuce, spinach, herbs). Eighteen inches is right for root crops (carrots, parsnips, beets) and any deep-rooted perennial like asparagus, rhubarb, or established fruiting bushes.
Bulk vs bagged: at landscape supply yards, bulk premium garden mix typically runs ~$50-90 per cubic yard delivered. A cubic yard equals about 36 of 0.75-cu-ft bags, or 27 of 1-cu-ft bags, or 18 of 1.5-cu-ft bags. By the time you’re past 1.5 yards (60+ bags), bulk wins on price and saves the lifting. For one or two beds, bags are easier — no delivery wait, and you can split the buying across weekend trips.
The 10% waste factor default covers spillage during transfer and minor spread loss. For new beds, bump to 15-20% — premium garden mix settles 15-20% in the first season as the peat and compost compress with watering. Fresh-filled beds always look a couple inches over the rim; mid-season they’re flat to the rim, and they need topping up the following spring. Plan for an annual top-off of 2-3 inches.
When to use a raised bed soil calculator
- New 4x8 vegetable bed — The standard 4 ft x 8 ft x 12 in raised bed takes about 1.3 cubic yards of premium soil mix. Output in bags matches retail-store pricing.
- Multi-bed garden plan — Three 3 ft x 6 ft beds at 18 inches deep. The calculator runs each bed; sum the results for the delivery quote.
- Topping off existing beds — Add 2-3 inches of fresh mix to settled beds before spring planting. The calculator handles the partial fill above existing soil.
How to use the Raised Bed Soil Calculator
- Measure the bed — For rectangular beds, multiply length by width. For an L-shape or keyhole bed, break into rectangles and sum. For round/oval beds, the calculator takes diameter directly.
- Pick a depth — Use 6 inches minimum for shallow beds (greens, herbs); 12 inches for most vegetables; 18 inches for root crops (carrots, parsnips) and any deep-rooted perennial. Topping off existing beds: 2-3 inches.
- Read the bag count — Output shows bags of the size you selected. Premium garden mix typically comes in 0.75-cu-ft, 1-cu-ft, or 1.5-cu-ft bags. Round up — partial bags don't store well outdoors.
- Apply waste factor — 10% covers spillage during transfer and minor spread loss. Bump to 15-20% for new beds — premium garden mix settles 15-20% in the first season as the compost compresses with watering.
Worked examples
4 ft x 8 ft bed at 12 in
Input: 4 ft x 8 ft x 12 in depth
Output: 1.30 cu yd / 47 of 0.75-cu-ft bags (with 10% waste)3 ft x 6 ft bed at 18 in (deep)
Input: 3 ft x 6 ft x 18 in depth
Output: 1.10 cu yd / 40 of 0.75-cu-ft bagsPremium garden mix is light and fluffy; settle is 15-20% in the first season.
Topping off, 4 ft x 8 ft x 3 in
Input: 4 ft x 8 ft x 3 in depth
Output: 0.33 cu yd / 12 of 0.75-cu-ft bags