How much soil do I need for a 4x8 raised bed?

Measure inside the boards, not outside: a 4' x 8' bed built from 2x6s is 93" x 45" inside. Two boards high (11") it holds 26.6 cu ft, 0.99 cu yd or 18 bags of 1.5 cu ft; three boards high (16 1/2") it holds 40.0 cu ft, 1.48 cu yd. Fill it to the top — new soil settles.

How many boards does a raised bed take?

Four per course: two long sides at the full length and two short sides fitted between them, shorter by two board thicknesses. A 4' x 8' bed two 2x6s high is 8 boards — 4 at 96" and 4 at 45", the short ones 2 from each 8' board.

Why is the bed a different height from the one I typed?

Because a bed is built in whole boards. Ask for 16" out of 5 1/2" boards and it takes 3 courses, which stack to 16 1/2". The calculator rounds up rather than down and says what the bed will actually be, and counts the soil to that height rather than to the number you typed.

Do I need stakes along the sides of a raised bed?

On a long bed, usually. Wet soil pushes outward, and a long side made of boards butted at the corners bows in the middle. A stake or post partway along any side longer than about 4' is the common rule of thumb — it is not a published standard, so the calculator keeps the spacing as a setting you can change. Taller beds and thinner boards want them closer.

Raised garden bed calculator: boards, posts and soil

Enter the size of the bed and the boards you are building it from. This works out how many courses that takes, the cut list, the posts, and how much soil fills it in cubic feet, cubic yards and bags.

Length

Outside — 96 for 8'

Width

Outside — 48 for 4'

Height

Rounded up to whole boards

Board width

= 5.5"

Advanced settings1 1/2" boards · a stake every 48" or less · 1.5 cu ft bags

Board thickness

= 1.5"

Most a side runs between posts

A rule of thumb, not a standard — 48" is the usual one, to stop a long side bowing

Bag size

cu ft

Printed on the bag you buy

2 courses, 11" high

Boards

4 long, 4 short

Soil

0.99 cu yd

26.6 cu ft

Bags

Posts

Built height

11"

Cut list

4 long sides at 96"The full outside length: 4 × 8 ft
4 short sides at 45"The width less two 1 1/2" boards, fitted between the long sides: 2 × 8 ft
6 posts at 11"4 inside the corners and 2 partway along the sides — add length if they go into the ground: 1 × 8 ft

Soil to fill it

Inside the boards11" deep, filled to the top — new soil settles: 93" × 45"
Cubic feet: 26.6
Cubic yardsWhat a landscape yard sells by: 0.99
BagsAt 1.5 cu ft a bag, rounded up: 18

Where to buy this

Long sides: 4 × 8 ftHome Depot Lowe's
Short sides: 2 × 8 ftHome Depot Lowe's
6 posts at 11"Home Depot Lowe's
18 bags of soilHome Depot Lowe's

This searches cedar because it is the usual choice for a bed; the board sizes are the ones you entered.

Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.

Worked example: a bed 8 ft × 4 ft

A 96" × 48" bed built from 5 1/2" boards is 2 courses high, 11": 8 boards — 4 at 96" and 4 at 45" fitted between them.

It stands on 6 posts of 11", 2 of them partway along the sides so no side runs more than 48" unsupported.

Inside it holds 26.6 cu ft of soil — 0.99 cu yd, or 18 bags of 1.5 cu ft.

Boards: 8
Height: 11"
Posts: 6
Soil: 0.99 cu yd
Bags: 18

These are the figures the calculator above opens on. Change any field and it works the same problem for your numbers.

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About raised garden beds

How much soil do I need for a 4x8 raised bed?: Measure inside the boards, not outside: a 4' x 8' bed built from 2x6s is 93" x 45" inside. Two boards high (11") it holds 26.6 cu ft, 0.99 cu yd or 18 bags of 1.5 cu ft; three boards high (16 1/2") it holds 40.0 cu ft, 1.48 cu yd. Fill it to the top — new soil settles.
How many boards does a raised bed take?: Four per course: two long sides at the full length and two short sides fitted between them, shorter by two board thicknesses. A 4' x 8' bed two 2x6s high is 8 boards — 4 at 96" and 4 at 45", the short ones 2 from each 8' board.
Why is the bed a different height from the one I typed?: Because a bed is built in whole boards. Ask for 16" out of 5 1/2" boards and it takes 3 courses, which stack to 16 1/2". The calculator rounds up rather than down and says what the bed will actually be, and counts the soil to that height rather than to the number you typed.
Do I need stakes along the sides of a raised bed?: On a long bed, usually. Wet soil pushes outward, and a long side made of boards butted at the corners bows in the middle. A stake or post partway along any side longer than about 4' is the common rule of thumb — it is not a published standard, so the calculator keeps the spacing as a setting you can change. Taller beds and thinner boards want them closer.