How to Build and Fill a Raised Bed

A raised bed is the most satisfying small build in a garden and the easiest one to under-order for. The bed comes out taller than planned, the soil to fill it is more than expected, and the plants go in at whatever spacing looks right on the day.

Four tools, in the order you meet them.

1. Build it — and note the real height

The Raised Bed Material Calculator gives you boards, and it flags something worth knowing before you buy soil: the bed will end up taller than the one you asked for, and will need more soil than you planned.

Board dimensions are nominal. Stack two boards described as a certain height and the finished bed is not twice that number — and since the bed's volume scales directly with its height, a small discrepancy in height is a not-small discrepancy in soil.

Build first, or at least settle your board stack, before ordering the fill.

2. Fill it

The Raised Bed Soil Calculator gives cubic feet, cubic yards and bags — three units because that is genuinely how it gets sold, and the crossover point matters.

Bags are convenient and expensive; a yard delivered loose is cheap and arrives as a pile on your driveway. Run both figures before deciding, especially for a whole plot rather than one bed.

3. Space the plants before you buy them

The Plant Spacing Calculator works out how many plants fit at your chosen spacing, laid out in rows, so you buy the right number instead of guessing.

The guessing usually goes one way: too many. Plants arrive at their final size, not their seedling size, and a bed planted to look full in May is a crowded, mildewy tangle by August. Buy to the spacing, then resist filling the gaps.

4. Compost: two numbers, one rule

The Compost Brown-to-Green Ratio Calculator clears up the most misquoted figure in gardening.

The famous 30:1 is carbon to nitrogen BY WEIGHT. What you actually stand there measuring is roughly 3:1 browns to greens BY VOLUME. Same rule, two completely different numbers — and people who have heard "30:1" and are adding thirty buckets of leaves to one bucket of kitchen scraps are building a pile that will never heat up.

If your compost is slow and dry, you are probably applying the weight ratio by volume. That is the single most common cause.

The short version

1. Settle the board stack first — the bed is taller than nominal, and volume follows height.
2. Price soil in bags and by the yard before ordering.
3. Buy plants to the spacing, then leave the gaps alone.
4. Compost is 3:1 by volume, not 30:1. The famous number is by weight.

Each tool has a printable, and the compost sheet is worth pinning up by the bin — the volume ratio is the one that gets forgotten between batches.