How this calculator works
Choose a hay allocation from an existing ration, or estimate the hay dry-matter allocation as a percentage of body weight. Enter only the hay portion, not total diet intake. Choose allocations appropriate to each group’s class and stage. The calculator neither selects an allocation nor checks nutritional adequacy. There is no automatic grain subtraction or cold-weather multiplier.
Dry matter is the part of hay remaining after water is excluded. Use a forage analysis where available, identify whether the dry-matter value is measured or estimated, and weigh several representative bales. The allocation is multiplied by animal count and days. Dividing that seasonal dry matter by the entered hay dry-matter fraction gives the consumed hay equivalent.
Feeding loss is a share of hay offered; storage loss is a share of incoming hay. To work backwards to a purchase, divide the consumed equivalent by the fraction retained after feeding, then by the fraction retained after storage. Enter losses for your feeding arrangement, hay lot and storage period. A percentage from a different animal species, feeder design or storage duration is not automatically applicable.
Combine all groups before rounding up to whole bales. Purchase weight, price and space use that whole-bale count. The ledger reconciles consumed hay, feeding loss, storage loss and rounding surplus at the same entered dry matter. It does not predict moisture changes, selective nutrient loss or whether weathered hay remains suitable. Already-stored or mixed hay lots need separate assessment.
In the worked example above, 10 kg of hay dry matter corresponds to approximately 11.11 kg of consumed hay per day at 90% dry matter, before losses. Loss allowances increase the purchase to 117 whole 20 kg bales for 180 days. This arithmetic does not select a ration.
Body-weight percentages depend on the horse’s class and diet. Extension Horses distinguishes forage from the total ration. Mississippi State Extension explains how storage conditions and duration affect losses. These publications support those distinctions, not every entered scenario. About & sources.
Entering hay already on hand subtracts whole bales from the rounded gross requirement and shows the net bales and cost still to buy. Check my season fields turn entered mid-season stock into an observed disappearance rate, a projected run-out date, and a whole-bale reorder quantity. The storage-upgrade scenario uses entered targets for seasonal savings and simple payback; supplier lead time adds an order-by date. Use the price converter, water planner and manure planner. The round-vs-square calculator compares explicit hay-DM purchases.
Barn-space volume uses Missouri Extension planning rates of 250 ft³/ton square and 310 ft³/ton round (US short tons); Extension Horses corroborates the square-hay rate. Actual needs vary with bale density, dimensions, and stacking; ventilation, access aisles, equipment turning space, wall clearance, and local fire-code space are not included.
Frequently asked questions
How much hay does a horse need per day?
Illustration only: for a 500 kg horse with an entered 2% body-weight hay-DM allocation, 10 kg dry matter/day equals approximately 11.11 kg hay at 90% dry matter, before losses. This is not a ration recommendation.
How many bales of hay does one horse need for winter?
Illustrative inputs: one 500 kg horse, 2% body-weight hay-DM/day, 180 days, 90% hay dry matter, 10% feeding loss, 5% storage loss and 20 kg bales give 117 whole bales. This is not a feeding recommendation.
Does feeding grain reduce how much hay a horse needs?
BaleMath does not automatically subtract grain or calculate a substitution ratio. Use the hay portion of an appropriate existing ration, and seek qualified advice when changing the diet.
Plan the rest of the barn
BaleMath is free to use. Numbers are planning estimates, not veterinary or nutritional advice.