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Dog Heat Cycle Calculator: When Is the Next Heat?

By Marcos Ortega · Updated · Runs in your browser; nothing you type is sent anywhere.

Educational estimate, not veterinary advice. These numbers come from published averages and can be wrong for your pet. Your veterinarian, who can examine your dog or cat, is the right person to decide what is healthy for them.

Dog heat cycle calculator

–next heat, average interval (7 months)

  • Proestrus
  • Estrus
  • Diestrus, then anestrus
  • Today
Next heat, usual range (4–13 months)
–
Proestrus, if average (9 days)
–
Estrus, if average (9 days)
–
Today
–

A female dog’s reproductive cycle has four stages. The first two, proestrus and estrus, are what owners call “heat”. Enter the first day of her last heat and the calculator gives the average dates of each stage and when the next heat is likely, using the figures in the Merck Veterinary Manual.

The four stages

Stage What happens How long (Merck)
Proestrus Bloody discharge and a swollen vulva; males are attracted but she won’t accept them yet 3 days to 3 weeks, 9 days on average
Estrus She accepts the male; the discharge may turn straw-colored; ovulation usually 1–3 days after estrus begins 3 days to 3 weeks, 9 days on average
Diestrus Heat is over; pregnancy, if she conceived 2–3 months if not pregnant
Anestrus Rest between cycles 1–6 months

The interval from one heat to the next is 4–13 months, 7 months on average (Merck). Cornell’s Riney Canine Health Center gives a similar picture: most dogs have two heats a year, every 5–11 months on average.

How the calculator counts

  • Proestrus: day 1 to day 9 (Merck’s average).
  • Estrus: day 10 to day 18 (another 9 days on average).
  • Next heat: 7 months after the start date (average), within a usual range of 4 to 13 months. We count the interval from the first day of one heat to the first day of the next.

Next heat (average) = first day of last heat + 7 months
Usual range = + 4 months … + 13 months

Worked examples

Example 1: her last heat started on March 10, 2026. Proestrus, if average: Tue, Mar 10, 2026 – Wed, Mar 18, 2026. Estrus, if average: Thu, Mar 19, 2026 – Fri, Mar 27, 2026. Next heat on average: Sat, Oct 10, 2026, usual range Fri, Jul 10, 2026 – Sat, Apr 10, 2027.

Example 2: last heat started on January 31, 2027. Next heat on average: + 7 months = Tue, Aug 31, 2027. Usual range: Mon, May 31, 2027 – Tue, Feb 29, 2028. (Adding 13 months lands in February, which has no 31st, so the calculator uses the last day of the month.)

Limitations

  • Averages, not her calendar. Every stage can be three times shorter or longer than the average. After two or three heats, her own pattern is a better guide than any average.
  • Not a fertility planner. Counting days can’t tell you when she ovulates. If you plan a breeding, your vet can time it with progesterone tests; if you want to avoid one, keep her away from intact males for the whole heat.
  • Size and breed matter. First heats come earlier in small breeds and later in large ones, and some breeds such as the Basenji typically cycle once a year.
  • Changes are worth a vet visit. Heats that stop, come much more often, or come with unusual discharge, drinking or lethargy should be checked.

Frequently asked questions

How often do dogs go into heat?

The Merck Veterinary Manual gives an interval of 4–13 months between heats, 7 months on average. Cornell's Riney Canine Health Center says most dogs have two heats a year, on average every 5–11 months, and that some breeds, such as the Basenji and Tibetan Mastiff, typically cycle once a year.

How long does a dog stay in heat?

The visible part, proestrus plus estrus, averages about 18 days in Merck's figures (9 + 9), but each stage can last from 3 days to 3 weeks. Merck's cycle table gives 7–10 days of proestrus bleeding.

When is a dog fertile?

During estrus. Merck's table says ovulation usually occurs 1–3 days after the onset of estrus. The day varies a lot between dogs, which is why breeders time it with progesterone tests at the vet rather than by counting days.

At what age is a dog's first heat?

Between 6 and 24 months, earlier in smaller breeds and later in larger breeds (Merck; Cornell).

Does spaying stop heat cycles?

Yes. Cornell notes that spaying is the main way to prevent heat cycles and pregnancy, and that it also prevents pyometra, a common infection of the uterus. Talk to your vet about timing; Cornell says it is best to wait about two months after a heat.

Sources

  1. Merck Veterinary Manual (Davidson AP). Breeding Management of Bitches. Checked October 25, 2026.
  2. Merck Veterinary Manual. Features of the Reproductive Cycle (table). Checked October 25, 2026.
  3. Cornell University College of Veterinary Medicine, Riney Canine Health Center. Dog Estrous Cycles. Checked October 25, 2026.

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