Garden Bed Calculator
Everything a new bed needs in one pass: edging, mulch or soil volume, and the plants it holds.
A new bed generates four shopping questions at once (how much edging, how much mulch or soil, how many plants, what does the edging cost) and most gardeners answer them in four separate guesses. This calculator answers them together from the bed’s dimensions, because they all derive from the same two numbers: its perimeter and its area.
Set the bed size, say whether you are mulching a ground bed or filling a raised one, and price the edging. Deep dives live in the specialists: Mulch Calculator, Raised Bed Soil Calculator and the Plant Spacing Calculator.
How this is worked out
Area = length x width; edging = the perimeter, 2 x (length + width), the figure everyone underestimates on the first bed. Volume = area x depth, with mulch priced at 3 in (the working depth for weed suppression) and a raised fill at 11 in (a standard two course bed). Bag counts use the two standard bag sizes, 2 and 3 cu ft, and the bulk threshold note is real: past one cubic yard (27 cu ft), a delivered load beats bags on price almost everywhere. Plant count reuses the standard spacing grid.
Assumptions:
- Rectangular beds. For an L or a curve, split it into rectangles for the area and walk the perimeter with a tape for edging; curves always take more edging than the formula suggests.
- Depths are settled depths. Loose material compacts about 10 percent in the first weeks, hence the standing advice to add a tenth.
- Edging price is per linear foot of material alone; posts, stakes and corners live in the same small change category as the hardware on the build cost page.
Worked examples
1. A 10 x 4 ft border refresh. Perimeter 28 ft of edging (about $84 at $3 per ft), mulch at 3 in: 10 cu ft, about 0.37 yards, so five 2 cu ft bags. Small numbers, one trip, no second journey for ‘one more bag’, which is the actual point of calculating.
2. Filling the same bed as a raised bed. At 11 in: about 37 cu ft, 1.36 yards. That crosses the bulk line: order 1.5 yards delivered and skip the nineteen bags.
3. A 20 x 3 ft hedge line bed. Edging jumps to 46 ft while the area stays modest: long thin beds are edging projects with a mulch footnote, and the calculator makes the proportion visible before the till does.
Mulch or fill: choosing the job
| Situation | Job | Depth used |
|---|---|---|
| Existing bed, weeds and water loss | Mulch | 3 in (2 in for fine bark near small plants) |
| New bed on dug soil | Mulch after planting | 3 in, kept clear of stems |
| New raised frame | Fill | Frame depth minus 1 in of freeboard |
| Topping up a settled raised bed | Fill (partial) | Measure the actual drop; usually 2 to 3 in a year |
The yearly top-up row is the one returning gardeners need most: raised beds subside as their organic matter digests, and 2 to 3 in a year is normal, not a defect.
Common mistakes
- Buying edging by area. Edging follows the perimeter, and on long thin beds the perimeter dwarfs intuition. Measure the outline, not the square footage.
- Bags for a bulk job. Past a cubic yard, bagged material costs two to three times the delivered price and generates a plastic pile. The calculator’s threshold note is a money line.
- No settling allowance. A bed filled exactly to the brim is a bed an inch low by June. Add the tenth, or plan the top-up.
- Mulch against stems. The last 2 to 3 inches around each plant stay bare. Mulch volcanoes rot crowns and bark with impressive reliability.
Putting the result to work
Take the four numbers (edging feet, yards or bags, plant count, rough edging cost) as the shopping list in one line. For depth variants and bagged versus bulk mulch pricing, the Mulch Calculator goes deeper; for the frame itself, the Raised Bed Build Cost Calculator prices the lumber.
What does my new bed need?
- Measure the bed’s length and width.
- Choose the job: mulch at 3 inches, or fill a raised bed at 11 inches.
- Pick the crop for the plant count, and price edging per foot if you want it costed.
- Read edging length, volume in yards and bags, and plant count in one result.
- Add 10 percent volume for settling, and switch to bulk delivery past one cubic yard.
How do I calculate edging for a bed?
Edging follows the perimeter: twice the length plus twice the width for a rectangle. Curves and L shapes take more than any formula, so walk them with a tape or a hosepipe laid along the line. Buy a few feet over; running one length short is the standard edging story.
When is bulk soil or mulch cheaper than bags?
At about one cubic yard (27 cubic feet) delivered bulk almost always beats bags, often by half. Below half a yard, bags win on convenience and leftovers stay clean and stored. The calculator prints both units precisely so this decision is arithmetic, not vibes.
How deep should mulch be?
About 3 inches for bark and wood mulches on beds (enough to suppress weeds and hold moisture), 2 inches for fine materials and around small or shallow rooted plants, and always kept a few inches clear of stems and trunks. Topping up annually by an inch maintains it as the lower layer digests.
Why does my raised bed sink every year?
Its contents are digesting: the compost and organic fraction of any bed mix decomposes, and the whole fill consolidates as it wets and dries. Two to three inches a year is normal. Top up in spring with compost or bed mix; the sink is the soil food web doing its work, not a construction failure.
Can this calculator handle a round bed?
Approximate it as a square of the same diameter for area and volume (close enough for ordering, then add the 10 percent), but measure the edging with a tape around the true circle: a circle’s edge is its diameter times about 3.14, which the rectangle’s perimeter overstates by a quarter.
Volumes are bed geometry at standard depths (mulch 3 in, raised fill 11 in). Bag sizes are the standard retail 2 and 3 cu ft. Plant counts use the standard spacing grid.
