How to use the heat cycle calculator
- Enter the first day of her last heat: the day you first noticed swelling or bloody discharge.
- Enter how many months apart her heats are. If you do not know, leave the Merck average of 7 months; if you have dates from earlier heats, use her own average.
- Read the next heat date, her likely fertile days and the next four cycles.
The formula
Next heat = start of last heat + interval. The calculator uses an average month of 30.44 days.
Inside each heat (Merck Veterinary Manual, pet owner version):
- Proestrus: about 9 days (from 2 days to 3 weeks). Swelling and a bloody discharge; males are interested but she will not mate.
- Estrus: about 9 days (from 3 days to 3 weeks). She accepts the male, the discharge often turns straw-coloured, and ovulation happens about 2–3 days after the hormone peak.
- The whole visible heat is usually 14–21 days (PetMD).
So the calculator marks days 9 to 18 as the likely fertile window and day 21 as the usual end. These are averages; your dog may be earlier or later.
Tips
- Keep a diary. Write down the first day of every heat. After two or three cycles you will know her own interval.
- Planning a litter? Talk to your vet about progesterone testing to find the best mating days, then use the dog pregnancy calculator for the due date.
- Not planning a litter? Keep her on a lead and away from intact males for the whole heat, and ask your vet about spaying.
- Heat changes with age. Very irregular, very long or very frequent heats are worth a vet check.
Sources
- Merck Veterinary Manual (pet owner version): Management of Reproduction in Dogs.
- PetMD: Dog in Heat (cycle length and age at first heat).
Frequently asked questions
How long is a dog in heat?
Usually two to three weeks. PetMD gives 14–21 days for a typical heat. Merck splits it into proestrus, about 9 days (from 2 days to 3 weeks), when she attracts males but will not mate, and estrus, about 9 days (from 3 days to 3 weeks), when she is receptive and ovulates.
How often do dogs go into heat?
About every six to seven months on average. PetMD says most female dogs come into heat about every six months, and the Merck Veterinary Manual gives an average interval of 7 months, with some large breeds such as Great Danes cycling every 9–12 months.
When will my dog go into heat for the first time?
Usually between 6 and 24 months of age. PetMD notes that small breeds often have their first heat at about 6 months, while large and giant breeds can be much later.
When will my dog go into heat again after having puppies?
Usually on her normal schedule, counted from the start of her previous heat, but it varies between dogs. Ask your vet, and keep her away from intact males if you do not want another litter.
How do I know when my dog's heat is over?
The discharge stops, the swelling of the vulva goes down, and she stops showing interest in males or letting them near her. Most heats are over within about three weeks (PetMD). Keep her away from males for a few more days to be safe.