Why the count runs away from the height
The intuition everybody starts with is that twice the height is twice the bags. It is not, for a reason that has nothing to do with hydraulics: a stack of loose bags cannot get taller without getting wider at the bottom, so raising the barrier adds courses and adds rows to the courses at the same time.
Run the default figures and a 40 foot barrier at 2 feet high with three rows across the base comes to 360 bags. Take the same barrier to 4 feet and, with the base widened in proportion, it is well past double. The ladder at the bottom of the results does this arithmetic at half, one, one and a half and twice your height, with the base width scaled alongside, so the shape of the curve is visible rather than assumed.
That is the number worth having before anybody starts. Three hundred and ninety-six bags including the spare allowance, at twenty an hour per person, is close to twenty person-hours of filling alone, before a single one is carried or placed, and filling is usually a two-person job.
What the geometry actually is
| Quantity | How it is worked out |
|---|---|
| Bags along a row | Barrier length divided by the filled bag length, rounded up |
| Courses | Finished height divided by the filled bag height, rounded up |
| Rows in each course | Tapering evenly from your base row count down to one at the top |
| Sand per bag | The three filled dimensions multiplied together |
| Base width | Base rows multiplied by the filled bag width |
Everything above rests on the filled bag dimensions, which are the dimensions of the bag as it lies once it has been filled and flattened, not the printed size of the empty bag. They vary with how full the bags are, which varies with who is filling them and how tired they are, and the three fields exist so you can put in what you are actually getting. Fill ten, lay them down, measure, and the rest of the page becomes considerably more accurate.
Sand, weight and where it stands
The default barrier holds about 133 cubic feet of sand, which is a shade under five cubic yards and, at 100 pounds per cubic foot, weighs a little under seven tons. Two things follow. The delivery is a real one and needs somewhere to be tipped that is not where the barrier is going. And close to seven tons standing on a strip of ground a hundred square feet in area is a bearing question if that ground is a deck, a paved area over something, or anything other than dirt.
Sand density moves with moisture, and wet sand is meaningfully heavier than the figure in the field. If a supplier gives you a weight per yard, use theirs. Excavation and bulk material volumes generally are handled by the excavation volume calculator, and site drainage measures of the more permanent kind are in the erosion and sediment control calculator.
The hours are the part that gets underestimated
Filling rate is per person-hour of finished bags, and it is generous at twenty. Filling is realistically a pair — one holding, one shovelling — and it slows down considerably after the first hour because it is heavy repetitive lifting from ground level, usually in rain, usually with a deadline. The calculator reports person-hours and then divides by your crew, and it explicitly excludes carrying, placing and stacking, which is a comparable amount of work again.
If bags and sand are being distributed by a local agency, that changes the arithmetic completely — they are often supplied pre-filled or filled on site, and there may be a limit per household. That is worth finding out before planning a day of shovelling.
What this page does not claim
It does not say that a barrier will hold water back, that any height or base width is appropriate, or that building one makes anywhere safe. Loose bags leak by design, water finds ends and gaps, and a barrier that diverts water somewhere else has consequences for whoever is downstream. The hazard that actually kills people during a flood is moving water, on foot and in vehicles, and both depth and speed are misjudged routinely.
Where a barrier goes, how it is built, and whether it is worth building at all belong to your local emergency management agency and your floodplain administrator. Whether to leave belongs to whoever issues the order, and what officials consistently ask is that people who are going to go, go early. The elevation side of the problem is on the flood elevation and freeboard calculator, and general preparation reading is in the storm and flood preparedness guide.
Questions people ask
How many sandbags do I need for a 40 foot barrier?
With the default assumptions on this page — 16 inch filled bags, 4 inch courses, three rows across the base tapering to one at the top — a 40 foot run 2 feet high comes to 360 bags, plus a spare allowance. Change any of the bag dimensions or the base rows and the answer moves substantially, which is why they are all inputs. What the page will not tell you is what height or base width is appropriate for your situation, because that depends on things a calculator cannot know and it belongs to your local emergency management agency.
Why does doubling the height more than double the bags?
Because a stack of loose bags has to get wider as it gets taller. Adding height adds courses, and it also adds rows across the base, so both dimensions of the cross-section grow together. The height ladder in the results scales the base width in proportion so you can see the effect directly. If you force a single row all the way up by choosing that profile, the count does scale with the height alone — and that shape is a different structure with different behaviour, which is precisely the point.
How much does a filled sandbag weigh?
It follows from the filled dimensions and the sand density, both of which you set. At the defaults — 16 by 10 by 4 inches of sand at 100 pounds per cubic foot — a bag comes out around 37 pounds, and the default barrier weighs something over six tons in total. Wet sand is heavier than the default figure, sometimes considerably, so if your supplier gives a weight per yard use theirs. The per-bag figure is the one that matters for whoever is lifting them, and the total is the one that matters for whatever they are standing on.
How long does filling take?
The calculator uses finished bags per person-hour, defaulting to twenty, and divides the total by your crew size. That figure covers filling only. Carrying, placing and stacking is a comparable amount of work again and is not included. In practice filling is a pair operation — one holding, one shovelling — and the rate falls off after the first hour because it is heavy repeated lifting from ground level, usually in bad weather with a deadline. If bags are being supplied pre-filled by a local agency, none of this arithmetic applies.
Will a sandbag barrier keep water out?
This page makes no such claim and cannot. Loose bags leak, water finds ends and gaps, and a barrier that diverts water successfully has diverted it somewhere, which is somebody else problem. What is being calculated here is quantities: bags, sand, weight, base width and hours. Whether a barrier is worth building where you are, how high, how it is built and what it is built against belongs to your local emergency management agency and your floodplain administrator, and the decision about leaving belongs to whoever issues the order.