Mare Heat Cycle Calculator
The first day of a mare's last heat goes in. Her next six projected heats come back, each with the likely ovulation window at the close of it, so a breeding shed can see which weeks of the season are live before the teasing chart fills in.
Day one of standing heat, off the teasing chart or the vet's exam. The projection counts forward from that date.
First day of one heat to the first day of the next. In season most mares run about 21 days.
Most mares show heat for about 5 to 7 days.
A projection is a guess until the real dates are written down. HorseBook keeps heat checks, covers, pregnancy checks and foaling on the mare they belong to, so next season's projection comes off her own history instead of the 21-day average. Every plan includes the mare side, from a three-mare band to eighty.
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What the calculator counts
A mare in the breeding season cycles on a roughly 21-day interval, measured from the first day of one heat to the first day of the next. Standing heat itself takes up about 5 to 7 days of that interval, and the rest is diestrus, when she wants nothing to do with the stallion. The calculator takes one date, adds the cycle length six times, and marks the heat that opens on each of those days.
Both numbers are editable because a band does not run on one number. A mare who came back in 19 days last season is a 19-day mare on this page, and a mare who holds a heat for nine days in April gets a nine-day heat. The projection is only as good as the interval fed into it, and her own interval beats the herd average every time.
The heat, the ovulation window, and why breeding runs late in it
Ovulation does not sit in the middle of the heat. It typically lands in the last day or two, which is why the table marks the closing 48 hours of every projected heat as the likely ovulation window rather than shading the whole heat evenly. It is also why covers and inseminations are timed late, and why a vet scanning a follicle carries more weight than any calendar.
For a stallion owner shipping cooled semen, that window is the logistics problem in one line: the collection, the courier, and the mare all have to arrive inside it. For a mare owner hauling to a cover, it is the difference between one trip and three. Projecting the window a month out is what makes either plan bookable.
Where the arithmetic breaks
A projection built from one date breaks in four predictable places, and an operation that knows them uses the table for planning rather than for decisions:
- The transitional season. Heats in early spring run long and irregular as mares come out of winter anestrus, so the interval is unreliable before the season settles.
- Foal heat. The first heat after foaling is counted off the foaling date, not off the old cycle, so a foaling resets the whole projection.
- She settled. A mare in foal stops cycling. Once a pregnancy check comes back positive, the dates that matter move to the horse gestation calculator.
- Individual variation. Cycle length shifts with the mare, the month, and the year. Two recorded seasons say more about her than any published average.
Educational, not veterinary advice. Ovulation is placed by examination, not by arithmetic. Breeding decisions on a mare belong with the veterinarian who is examining her.
Common questions
How often does a mare come into heat?
In the breeding season a mare comes into heat about every 21 days, counted from the first day of one heat to the first day of the next. Individual mares run shorter or longer, commonly somewhere between 18 and 26 days, and the same mare can vary across a season. That interval is what the projection above rests on, which is why it is worth replacing with her own recorded dates once an operation has a season of them.
How long is a mare in heat?
Most mares show heat for about 5 to 7 days. Across a band of mares anything from 3 to 10 days turns up, and heats early in the season often run longer and read less cleanly. The calculator defaults to 6 days and accepts 3 to 10, because the length of standing heat is what places the ovulation window at the end of it.
When does a mare ovulate?
Ovulation typically falls in the last day or two of the heat rather than the middle of it. That is why breeding is timed late in the heat, and why a vet scans the follicle by ultrasound instead of counting days. The follicle says how close she is; the calendar only says how close she usually is.
What is foal heat?
Foal heat is the first heat after foaling, and it often arrives within the first couple of weeks. Whether to breed on it is a call for the veterinarian who has examined the mare, not a rule that applies across a band. A foaling also resets the arithmetic, so the projection above does not carry across it.
Why do mares not cycle in winter?
Mares are seasonally polyestrous, which means they cycle with increasing daylight. The breeding season runs through spring and summer, and most mares stop cycling in the short days of winter. Between the two sits the transitional season in spring, where heats are long, irregular, and hard to read off a chart. Lights programs are used to bring mares forward earlier than daylight would; the specifics of running one belong with the vet who will manage it.
How does an operation track a mare's heat cycle?
Three sources, kept together. A teasing chart with a date and a score every time she is teased. The behavior anyone in the barn can see, from winking and squatting to standing for the stallion. And the vet ultrasound when a cover is close. The fourth part is the one that gets skipped: keeping those dates after the season ends. A mare with two seasons of recorded heats gets a projection built on her own interval instead of the herd average.
Does this calculate the foaling date?
No. This tool projects heats forward from a heat date. A foaling window is counted from the cover date instead, which is a separate calculation: the horse gestation calculator marks the day-320 safe date, the 340-day average, and the day-350 upper edge of normal.
Keep the teasing chart, not just the projection
Next season's projection is only real if this season's dates were written down. Three guides cover what that record holds and what it is worth at scale:
- A mare's season in HorseBook: heat checks to a foal on the ground walks one mare from the first heat check through the foaling record.
- Horse breeding documentation: what to record at every stage covers heat cycles, ultrasounds, semen evaluation, stallion contracts, and registry rules.
- Broodmare management app: every mare in the band on one board covers what a board has to answer at 10, 30, or 80 mares.
Every mare's cycle on one board
HorseBook records heat checks against the mare, alongside her covers, pregnancy checks, foaling window and progeny, and the Broodmare Board reads the whole band at once: open, served, or in foal. Mare-side reproduction is on every plan, which is why a three-mare band and a thirty-mare one see the same screen.
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