Why bale weight has to be estimated at all
Hay is bought by the bale and fed by the ton, and nobody puts a round baler over a scale. So the standard method is geometry: work out the bale's volume from its dimensions, multiply by the density of the forage packed inside it, and you have the weight. For a round bale the volume is a cylinder, (π/4)·D²·W; for a large square it is simply length × width × height.
The consequence people miss is that weight goes with the square of the diameter but only linearly with the width. Step a 4 ft bale up to a 5 ft bale at the same width and density and the volume rises by 5²/4² = 1.5625, so the bale is 56% heavier. Step the width from 4 ft to 5 ft instead and the volume rises by 5/4 = 1.25, a 25% increase. That is why a load of "round bales" quoted without dimensions is not a quantity of hay, and why a per-bale price is meaningless until it is converted to a per-ton price.
The second consequence is that moisture is part of the weight. A bale at 82% dry matter carries nearly a fifth of its weight in water, and water is not feed. Comparing a dry bale at 88% with wrapped baleage at 55% on an as-baled price is comparing two different products. The only fair comparison is dollars per ton of dry matter, which this calculator gives you alongside the as-baled figure.
The three numbers behind the estimate
Dimensions. Measure the bale, do not trust the baler's badge. Round bales settle, sag on the bottom when stored on the ground, and shrink slightly as they dry, so a nominal 5×6 is rarely exactly 5 ft by 6 ft after a month in the yard. Measure the diameter across the middle of the round face and the width along the axle. Be careful with naming conventions too: a "4×5" means 4 ft wide by 5 ft in diameter in some regions and the reverse in others, which is a 25% difference in weight. Enter the two dimensions in the labelled fields rather than relying on the shorthand.
Density. This is where the uncertainty lives. Density depends on the baler, the pressure setting, the forage species, the length of chop and how dry the crop was when it went in. A variable-chamber baler also packs the core differently from the outer wraps, so any single number here is an average over the whole bale rather than a physical property of the hay. The right way to use this calculator is to weigh one representative bale once, divide the scale weight by the volume the calculator shows in its steps, and use that figure from then on for that baler and that crop.
Dry matter. Dry matter is 100 minus moisture. A moisture probe in the bale gives a workable field number; a laboratory dry-down gives the real one. Baled dry hay generally lands in the high 80s, and anything materially wetter than that is either going to heat in storage or is intentionally wrapped as baleage. The dry matter figure does not change the bale's weight — it changes how much of that weight is feed.
Worked example: fifty 5 ft by 4 ft bales at $60 each
You are looking at fifty round bales measured at 5 ft diameter and 4 ft wide, baled dry at 88% dry matter, offered at $60 a bale delivered. Your baler weighs out at about 10 lb per cubic foot on this grass hay.
- Volume. (π ÷ 4) × 5² × 4 = 0.785398 × 25 × 4 = 78.540 ft³.
- Weight as baled. 78.540 × 10 = 785.4 lb per bale.
- Dry matter per bale. 785.4 × 0.88 = 691.2 lb.
- Tons in the load. 50 × 785.4 ÷ 2,000 = 19.63 tons as baled, and 50 × 691.2 ÷ 2,000 = 17.28 tons of dry matter.
- Cost per ton as baled. Each bale is 785.4 ÷ 2,000 = 0.3927 ton, so $60 ÷ 0.3927 = $152.79 a ton.
- Cost per ton of dry matter. Each bale carries 691.2 ÷ 2,000 = 0.3456 ton of dry matter, so $60 ÷ 0.3456 = $173.62 a ton of dry matter.
Now compare a neighbour's 5 ft by 5 ft bales at $70. Their volume is (π ÷ 4) × 25 × 5 = 98.175 ft³, so at the same density they weigh 981.7 lb, or 0.4909 ton. That is $70 ÷ 0.4909 = $142.60 a ton — cheaper hay despite the higher sticker price, because the extra foot of width added 25% more forage for a 17% higher price.
How to use the number without over-trusting it
Treat the output as an estimate with a real error bar, and know which direction each error runs. Bales stored outside on the ground lose dry matter from the bottom third and take on water in the outer wraps, so a weathered bale can weigh close to its original figure while containing materially less feed. Bales that sagged in storage are no longer circular, and measuring the flattened diameter overstates the volume of the true cross-section.
The density figure is where the estimate is most sensitive, and the relationship is exactly proportional: a 10% error in density is a 10% error in weight and a 10% error in every ton and dollar figure downstream. That is a strong argument for the one-time calibration weighing. If you have no scale, a livestock scale at a sale barn, a farm truck at a certified scale before and after unloading, or a loader bucket scale will all get you close enough to be worth far more than a book density.
For feeding decisions, use the dry matter figure. Feed the ton-of-dry-matter number into the hay bales needed calculator or the dry matter intake calculator and you will size a winter's hay honestly. Feeding losses at the ring are a separate allowance on top and are frequently larger than the difference between any two density assumptions you might argue about.
Round bale weight by size and density
| Bale (D × W) | Volume | 8 lb/ft³ | 10 lb/ft³ | 12 lb/ft³ |
|---|---|---|---|---|
| 4 × 4 ft | 50.27 ft³ | 402 lb | 503 lb | 603 lb |
| 4 × 5 ft | 62.83 ft³ | 503 lb | 628 lb | 754 lb |
| 5 × 4 ft | 78.54 ft³ | 628 lb | 785 lb | 942 lb |
| 5 × 5 ft | 98.17 ft³ | 785 lb | 982 lb | 1,178 lb |
| 5 × 6 ft | 117.81 ft³ | 942 lb | 1,178 lb | 1,414 lb |
| 6 × 5 ft | 141.37 ft³ | 1,131 lb | 1,414 lb | 1,696 lb |
| 6 × 6 ft | 169.65 ft³ | 1,357 lb | 1,696 lb | 2,036 lb |
As-baled weights, so they include the water. Notice that a 5 × 6 and a 6 × 5 differ by 20% at the same density even though both are called "a five by six" in conversation.
Where bale-weight estimates go wrong
- Swapping diameter and width. A 5 × 6 and a 6 × 5 are not the same bale. Diameter enters squared, so the taller-diameter bale always weighs more at equal density.
- Using a book density instead of a scale. Density is the only input that cannot be measured with a tape, and weight is directly proportional to it. Calibrate once and the estimate becomes genuinely useful.
- Comparing wet and dry bales on the as-baled price. Baleage at 55% dry matter is nearly half water. Compare on dollars per ton of dry matter.
- Measuring the nominal size rather than the bale. Bales settle, sag and shrink. Measure a few in the stack and average them.
- Ignoring storage and feeding loss. This calculates what is in the bale on the day it was made, not what reaches the animal. Outside storage on bare ground and open ring feeders both take a share.
- Assuming uniform density. Variable-chamber balers build a softer core; fixed-chamber balers do the reverse. The figure here is a whole-bale average, which is the correct thing to use for a weight but not a description of any one spot in the bale.
Where bale weight fits in the rest of the hay decision
Bale weight is the hinge between three separate questions. On the production side, tons per acre off the field comes from bale count times bale weight; the hay tonnage per acre calculator closes that loop. On the buying side, the per-ton figures here are what let you compare a per-bale offer against a hay auction report, which is always quoted per ton. On the feeding side, dry matter per bale divided by daily dry matter intake gives feeding days per bale, which is what actually determines whether the stack lasts until spring.
Two adjacent tools finish the job. The hay bales needed calculator works out how many bales a group requires over a winter, and the pasture grazing days calculator tells you how much of that winter you can avoid feeding altogether. If you are pricing standing hay rather than baled, remember that a per-acre price has to pass through both a yield estimate and this weight estimate before it becomes comparable.
One last note on units: everything here is in short tons of 2,000 lb, which is the standard hay trading unit in the United States. Metric hay is traded per tonne of 1,000 kg, which is 2,204.6 lb, so a price per tonne is about 10.2% higher than the same price per short ton for the same hay.