Work out daily forage from bodyweight, then how much hay you need per week and for the whole winter, for one horse or the whole yard. Choose the forage percentage, how much grazing there is, and your bale size: small square, large square, round or your own weight.
Hay is usually 85–90% dry matter; the figures are as-fed weights. Haylage holds more water, so more is needed by weight. Weigh a net rather than guessing.
The calculator starts from your country's typical bales: a UK or Irish small bale of about 20 kg, a US or Canadian small square of about 55 lb (25 kg) and large round of about 1,000 lb, and in Australia and New Zealand small squares of about 25 kg and rounds of about 300 kg. Weights are shown in pounds in the US. In parts of northern Europe haylage or silage is fed more than hay; use the weight of what you buy.
Most horses need about 2% of their bodyweight in forage a day, so a 500 kg horse needs about 10 kg. Grazing provides part of this when they are turned out.
Multiply the daily hay per horse by the number of horses and the days of winter, then divide by your bale weight. One 500 kg horse on 10 kg a day for 180 days needs about 1,800 kg, or 90 small bales of 20 kg.
A minimum of about 1.5% of bodyweight a day as dry matter is commonly advised, even for horses on a weight-loss plan.
A planning guide for healthy adult horses; ask your vet or a nutritionist about horses with medical conditions.
Related: horse weight calculator · grazing calculator · cost of keeping a horse.
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