Solve backwards
How long a flood barrier will my sandbags build?
- 100 sandbagsThe example worked below
- Sandbag CalculatorEvery figure is its own
You have
How much sandbags?
Worked by the Sandbag Calculator at the settings below.
The settings it uses
- Number of Courses (Layers)
- 2
What it will do
Enough for 50 ft of barrier.
One barrier. Enough for 50 ft of barrier.
Or several. About 1 barrier of the size the Sandbag Calculator opens on: 49 ft of barrier.
Check it forwards
- Barrier Length
- 50 ft
- It asks for
- 100 sandbags
- You have
- 100 sandbags
The Sandbag Calculator asks for 100 sandbags at that size, and 101 sandbags one step larger.
If you had more, or less
Around your amount
Each worked by the Sandbag Calculator at the same settings.
Half
50 sandbags
25 ft of barrier
Yours
100 sandbags
50 ft of barrier
Half as much again
150 sandbags
75 ft of barrier
Double
200 sandbags
100 ft of barrier
Common amounts
What each amount of sandbags covers
The sizes it is usually sold in, each worked by the Sandbag Calculator at the settings above.
| You have | It covers | As one barrier | Average barriers |
|---|---|---|---|
| 50 sandbags | — | 25 ft of barrier | under 1 |
| 100 sandbags | — | 50 ft of barrier | 1 |
| 200 sandbags | — | 100 ft of barrier | 2 |
| 500 sandbags | — | 250 ft of barrier | 5 |
A foot a bag a course
The calculator counts a standard filled sandbag as about 1 linear foot (0.3 m) of wall per stacked course, so bags equal length times courses. A hundred bags build about 50 feet (15.2 m) of barrier two bags high, or about 33 feet three high. Two or three courses is common for a basic flood barrier; higher water needs more courses and a wider, stepped-back base for stability.
A barrier is a pyramid
The count is a single-width stack, one bag per foot in every course. A barrier that actually has to hold water is built as a stepped pyramid with a base several bags wide, so the real count is a multiple of this figure, and the multiple grows with the height. The ends have to climb onto ground standing above the water level, and the ends are where barriers of this kind usually fail.
Water finds other ways in
This is a bag count, not a flood defence design. Water seeps through and under a stack and can pipe through the ground beneath its toe, and a barrier stacked hard against a wall loads that wall. Water coming up through drains, air bricks and other below-ground routes is not stopped by any surface barrier. Nothing is added for bags that split, burst or get spent holding sheeting down.
How the answer is worked
Nothing here is a second formula. The Sandbag Calculator runs forwards — a size in, a quantity out — and this page grows its own starting barrier in proportion until the quantity it asks for is what you have. Where it rounds up to whole containers, the answer is the largest size before the next one would be needed.
Where the calculator adds an allowance for cuts or breakage when you order, the same allowance comes off here, because it is the same calculator: a stack you already own finishes less than its face value. Where it adds none, neither does this page — the settings beside the answer say which.
Sandbag Calculator
runs it forwardsEstimate how many sandbags you need to build a flood or erosion control barrier.
Work out what to buy insteadSolve backwards
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Solve backwards
Start from what is already in the yard
Paint, sheets, rolls, bags and pieces, each read the other way by the calculator that orders it.
The calculator's own figures
Nothing is estimated twice: the page runs the same calculator you would order with.
The largest size that fits
Where a calculator rounds up to whole containers, the answer stops before the next one.
Both unit systems
Gallons and litres, square feet and square metres, side by side.
Frequently asked questions
- How much barrier does 100 sandbags of sandbags cover?
- At the Sandbag Calculator's own settings: Enough for 50 ft of barrier. That is about 1 barrier of the size the calculator opens on: 49 ft of barrier. Every figure follows the settings, which you can change in the calculator.
- How many sandbags per foot of barrier?
- About one bag per linear foot (3.3 per metre) per course for a single-width stack, the calculator's figure, so two courses take two bags a foot. A barrier meant to hold water is built as a pyramid several bags wide at the base, which multiplies that count.
- How many sandbags to protect a door?
- A 3 foot (0.9 m) doorway two bags high takes about 6 bags in a single line, plus more to tie each end into the frame or the ground beside it. Anything beyond shallow water needs a wider base, and water can still arrive through drains and air bricks that the bags do not cover.
- How high should a sandbag barrier be?
- The calculator takes the number of courses as given and does not relate it to the water level or the freeboard above it. Two or three courses is common for a basic barrier, and anything higher needs a wider, stepped-back base. For real flood risk, follow the local emergency authority's guidance.