Dog Heat Cycle Calculator (next season & fertile phase estimate)

Understanding the dog heat cycle calculator — concept and principle

This calculator estimates when a female dog's next heat will start from the first day of her last heat and her usual interval, and shows the average timing of proestrus and estrus within the current heat. Owners use it to plan walks, boarding, shows and supervision, and breeders use it as a first rough guide.

Counting to the next heat: with the default inputs the result is May 12, 2027 (next heat expected).

How it is calculated

Next heat ≈ first day of last heat + usual interval in months × 30.4375 days (rounded).
Normal range for the next heat: 4 months (122 days) to 13 months (396 days) after the last one.
Estrus starts about day 9 and ends about day 18 after bleeding begins.
Days since the heat started = today − start date.
Stage: before day 9 proestrus, day 9 to 17 estrus, from day 18 after estrus.

The principle

Most female dogs come into heat about twice a year. The interval averages 7 months but normally ranges from 4 to 13 months, and some large breeds cycle every 9–12 months. Each heat starts with proestrus: the vulva swells, there is a bloody discharge, and the female attracts males but refuses them. This lasts about 9 days, but anywhere from 2 days to 3 weeks. Then comes estrus, the fertile phase when she accepts mating, also about 9 days on average (3 days to 3 weeks); ovulation happens during estrus. The calculator uses these averages, so real dogs may be days off.

Worked example

Default inputs: last heat started Oct 11, 2026, usual interval 7 months, viewed on Nov 10, 2026.

Next heat = Oct 11, 2026 + round(7 × 30.4375 = 213.06) = 213 days = Wed, May 12, 2027.
Earliest normal = + 122 days = Wed, Feb 10, 2027; latest normal = + 396 days = Thu, Nov 11, 2027.
Estrus about Tue, Oct 20, 2026 (day 9) to Thu, Oct 29, 2026 (day 18).
Days since the heat started = 30, so the stage is after estrus (out of season).

How to read the result

Use the next-heat date to start watching for swelling, licking and spotting a few weeks ahead. Keep a log of each heat: after two or three cycles your own dog's average interval is much more reliable than the population figure. The estrus dates are only averages; the fertile window can start earlier or end later.

Common mistakes

Using the dates to time a mating; vets use hormone tests and vaginal smears for that.
Relying on the "after estrus" stage as contraception.
Assuming young dogs have regular cycles; the first heats are often irregular.
Entering the date bleeding was noticed rather than when it began.

Keep in mind: Individual dogs vary widely. This is not a diagnosis. See your vet if heats are very frequent, very far apart, very long, or if there is discharge outside a heat.

Frequently asked questions

How often do dogs go into heat?

Usually about twice a year, every 7 months on average, with a normal range of 4–13 months.

How long does a dog's heat last?

Proestrus and estrus each last about 9 days on average, so roughly 2–3 weeks in total.

When is a dog fertile?

During estrus, on average from about day 9 after bleeding starts, but the timing varies a lot.

Sources: Merck Veterinary Manual (pet owner version), A. P. Davidson, "Management of Reproduction in Dogs"

Checked 9 Nov 2026. Merck Veterinary Manual (pet owner version), A. P. Davidson, “Management of Reproduction in Dogs”: estrous cycles usually twice a year, interval 4–13 months (average 7), large breeds such as Great Danes every 9–12 months; proestrus about 9 days (2 days–3 weeks); estrus about 9 days (3 days–3 weeks); ovulation during estrus.

Your numbers