Trench volume is width times depth times length, and width is the lever
Two hundred feet at 12 in wide and 42 in deep is 1 x 3.5 x 200 = 700 cubic feet, or 25.93 cubic yards. Cut the same trench 6 in wide with a chain trencher and it halves to 12.96. Depth is usually fixed by the frost line and is not yours to choose; width often is, and it is the only free variable in the whole excavation.
Then it swells. Soil dug out occupies more space than it did in the ground, typically a fifth to a third more for ordinary material, so 25.93 cubic yards in place becomes about 32.4 beside the trench. That matters for where the spoil goes, how much room there is to work, and how much has to be carted away if the backfill does not all go back — and it never all goes back, because the pipe, the bedding and the compaction take some of the space.
The water inside the pipe is the number that decides the freeze
A pipe of inside diameter d inches holds pi/4 x d squared / 144 cubic feet per foot, and at 7.48052 gallons per cubic foot that comes to 0.0408 x d squared gallons per foot. A 1 in line is 0.0408 gallons a foot; 200 feet of it plus two 3.5 ft risers is 8.45 gallons, weighing about 70 pounds.
| Inside diameter | Gallons per foot | Per 100 ft | Relative to 1 in |
|---|---|---|---|
| 1/2 in | 0.0102 | 1.02 gal | 0.25x |
| 3/4 in | 0.0229 | 2.29 gal | 0.56x |
| 1 in | 0.0408 | 4.08 gal | 1.00x |
| 1 1/4 in | 0.0637 | 6.37 gal | 1.56x |
| 1 1/2 in | 0.0918 | 9.18 gal | 2.25x |
Volume goes with the square of the diameter, so stepping up one nominal size for flow reasons raises the standing water by half again. That is not an argument against the larger pipe — flow usually wins — but it is the reason a line that has to be drained down each autumn is easier to drain when it is small, and the reason the standing volume is worth knowing before the winter rather than during it.
What drains back and what does not
Water runs downhill to the lowest point in the run and stops there. If the barn end is 4 ft above the low point, opening the line at the barn empties the rising leg back down to the low point — and that is where all of it now sits, in the one place with no drain. A line that is genuinely drainable falls continuously to a single point with something to open there. A line that merely rises at the far end is not drainable at all, it just moves the water to a different part of the pipe.
That is a layout decision made with the trencher running, not afterwards, which is why it is on this page rather than left to the plumbing.
Bedding and backfill have to add up
Four inches of sand under the pipe and four over it, in a 12 in wide trench over 200 ft, is 200 x 1 x 0.667 = 133 cubic feet, or 4.94 cubic yards of sand to buy. That comes out of the trench volume, so the native soil going back is 25.93 minus 4.94 minus the tiny volume of the pipe itself, 20.95 cubic yards. Against 32.4 cubic yards of loose spoil sitting beside the trench, that leaves a real surplus to spread or cart, and it is bigger than most people plan for.
Where this hands off
This page does volumes and quantities, not hydraulics. If the question is whether the pipe is big enough to deliver useful flow at the far end, the water flow and pressure drop calculator is the one that answers it, and over 200 ft of small pipe the answer is often no. For what the line has to supply once it gets there, the horse water trough calculator takes your own gallons per head and works out daily demand and refill intervals. If the trench is for drainage rather than supply, the French drain calculator and the drain slope calculator cover that instead.
Questions people ask
How deep does a water line to a barn have to be?
Deeper than the frost line where you are, and this page will not tell you what that is. Frost depth varies from a few inches to six feet across the country, the depth your adopted code requires is not always the same as the frost depth, and the water utility may require something else again at their end. The building department answers it in one phone call. The field defaults to 42 in only so the arithmetic has something to run on; treat it as a placeholder and replace it.
How many gallons are in 200 feet of 1 inch pipe?
About 8.16 gallons in the horizontal run, or 8.45 with two risers at 3.5 ft. The general figure is 0.0408 x d squared gallons per foot with d in inches inside diameter, which comes straight from pi/4 x d squared / 144 cubic feet per foot times 7.48052 gallons per cubic foot. That works for any pipe and it is worth remembering as roughly 4 gallons per hundred feet of 1 inch line.
How much bigger is the spoil pile than the trench?
A fifth to a third bigger for most soils, and the calculator defaults to a quarter. Sand swells least because it was already loose; clay swells most because it comes out in chunks with voids between them. On a 200 ft trench at 12 by 42 in that is 25.93 cubic yards in the ground becoming about 32.4 beside it. After the bedding sand and the pipe take their share, there is more spoil than there is room to put back, every time.
Can I just drain the line for winter?
Only if it falls continuously to one point that you can open. A line that rises to the barn does not drain when you open the barn end — the water simply runs back to the lowest point in the run and sits there, which is usually the middle of a field with nothing to open. The place to fix that is while the trench is being cut, by giving the whole run a consistent fall to a single low point with a drain at it. The calculator reports the standing volume so you know what you are trying to get rid of.
What about heat tape or a heated waterer instead?
That is an electrical installation in a wet outbuilding and it is outside what this page covers. What is worth saying plainly is that heating equipment around animals and bedding is one of the leading causes of barn fires, and that a heat tape on a buried line is a device you cannot inspect. What may be installed, on what circuit, with what protection and to what instructions is a question for a qualified electrician and for the product documentation, not for arithmetic about trenches.