About the Dog Heat Cycle Calculator
This dog heat cycle calculator predicts when an unspayed female dog will next come into season. Enter the first day of her last heat (the first day you saw swelling or bloody discharge), how many months usually pass between her heats and how long her heats last. It returns the date the next heat should start, the days she is most likely to be fertile, when that season should end, and a calendar of her next four predicted cycles.
Owners use it to plan walks, boarding and holidays, to keep her away from intact males at the right time, and to talk to their vet about the best time to spay. Breeders can use it to plan ahead, although the fertile window here is only a rough guide.
Most dogs come into heat about twice a year, with an average interval of about seven months and a normal range of four to twelve months; Basenjis and some sled-dog breeds cycle only once a year. A heat typically lasts two to three weeks, and VCA Animal Hospitals gives days 10–14 as the usual best breeding days — but some females ovulate as early as day 3 or as late as day 18. Progesterone blood tests are the only reliable way to time breeding, and a dog can get pregnant on any day of her heat.
How to use the dog heat cycle calculator
- 1Enter the first day of her most recent heat.
- 2Enter how many months usually pass between her heats (7 if you are unsure).
- 3Adjust the heat length if her seasons are shorter or longer than 3 weeks.
- 4Read the next start date and likely fertile window.
- 5Use the calendar to plan walks, boarding or a spay appointment.
Formula and method
The calculator adds your dog’s usual interval in calendar months to the first day of her last heat to predict the next one, and repeats this for the following cycles. The first day of heat counts as day 1, so the likely fertile window of days 10–14 is the start date plus 9 to 13 days, and the heat ends on the start date plus the heat length minus one day.
VCA Animal Hospitals reports an average interval of about seven months (range four to twelve), heats lasting about two to three weeks, the receptive phase starting around days 7–10 and the best breeding time usually between days 10 and 14. Individual dogs vary, cycles in young dogs are often irregular, and ovulation can happen as early as day 3 or as late as day 18 — so use the fertile window as a guide, not a guarantee.
- Interval
- Months between the start of one heat and the next
- Length
- Days from the first sign of heat to the end of discharge and swelling
Worked examples
Last heat 1 March 2026, 7-month cycle
Seven months after 1 March is 1 October 2026. Days 10–14 of that heat fall on 10–14 October, and a 21-day heat ends on 21 October. The next one after that is due around 1 May 2027 — about 1.7 heats a year.
Twice-a-year dog (6-month cycle)
A dog that last came into heat on 15 January and cycles every six months should start again around 15 July, with likely fertile days of 24–28 July and the season ending around 4 August.
Basenji with a yearly cycle
Basenjis typically come into heat once a year. Twelve months after 20 October 2025 is 20 October 2026, so her most fertile days are likely to be 29 October to 2 November.
Frequently asked questions
How often do dogs go into heat?+
Most dogs come into heat about twice a year. VCA Animal Hospitals gives an average interval of about seven months, with a normal range of four to twelve months. Basenjis and some sled-dog breeds usually cycle only once a year.
How long does a dog’s heat last?+
A heat (season) usually lasts about two to three weeks, starting with vulvar swelling or bloody discharge and ending when the discharge stops and the vulva returns to normal size.
When is a dog most fertile during her heat?+
Most females are most fertile between about days 10 and 14 of the heat, often when the discharge turns lighter and she starts accepting a male. Some ovulate as early as day 3 or as late as day 18, so progesterone testing is used to time breeding precisely.
At what age does a dog have her first heat?+
Usually at around 9–10 months, but small breeds can start as early as 4–6 months and giant breeds may not have a first heat until 18–24 months. First cycles are often irregular.
Can I spay my dog while she is in heat?+
Many vets prefer to wait until 2–3 months after a heat because the reproductive tract has a bigger blood supply during and just after a season, which raises surgical risk. Ask your vet about the best timing for your dog.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.