Dog heat cycle calculator

Enter the first day of your dog's last heat and how often she cycles. You get the window for her next heat, the dates of each stage and her most fertile days. For a puppy, switch to First heat.

Tim de JongUpdated October 2, 20266 min read

Her cycle

Last heat started

First day of swelling or bleeding

How often she cycles

Twice a year, about every 6 to 7 months

Next heat expected

–

Enter the first day of her last heat.

Predictions use average cycle lengths and can be weeks out for an individual dog. Never rely on them to prevent a pregnancy, and see your vet about any heat that seems unusual.

What are the four stages of a dog's heat cycle?

A dog's reproductive cycle has four stages. Only the first two are what owners call being "in heat"; the other two are quiet. The averages below are the ones the calculator uses, from the Merck Veterinary Manual:

Stage	Average	Range	What you see
Proestrus	9 days	3 days to 3 weeks	Swollen vulva, bloody discharge; attracts males but refuses them
Estrus	9 days	3 days to 3 weeks	Lighter, watery or pink discharge; accepts males; ovulates
Diestrus	2 to 3 months	pregnant or not	Refuses males; if she was mated, this is the pregnancy
Anestrus	1 to 6 months	varies most	Resting phase with no signs until the next proestrus

Worked example: a heat that starts on Mar 1, 2026 would, on average, be in proestrus until Mar 9 and in estrus from Mar 10 to Mar 18. If she cycles every 6 to 7 months, the next heat should start between Aug 31 and Sep 30, 2026.

How often do dogs go into heat?

Most dogs come into heat twice a year. The average gap from the start of one heat to the next is about 7 months, but Merck gives a normal range of 4 to 13 months, so a regular cycle of 5 or 10 months is not a problem in itself. Some small breeds cycle three times a year, while Basenjis, Tibetan Mastiffs and some giant breeds come into heat only once a year.

The best predictor is your own dog. Note the first day of every heat; after two or three, pick "Her own gap" in the calculator and enter her real interval. Young dogs are the exception: cycles can be irregular for up to two years before they settle.

When will my puppy have her first heat?

Smaller dogs mature sooner. Typical ages for a first heat by expected adult size:

Adult size	Weight	First heat
Small	under 20 lb (9 kg)	6 to 7 months
Medium	20 to 50 lb (9 to 23 kg)	7 to 9 months
Large	50 to 90 lb (23 to 41 kg)	12 to 16 months
Giant	over 90 lb (41 kg)	18 to 24 months

If you plan to spay her, the timing relative to her first heat is a decision to make with your vet. Our spay and neuter age calculator summarizes the research by breed and size.

When is a dog most fertile during her heat?

She ovulates in estrus, usually around the time her discharge turns from red to watery or pink. Her eggs then need another 2 to 3 days to mature before they can be fertilized. For most dogs that puts the best breeding days at around day 10 to 14 of the heat, counting from the first day of bleeding, and most won't accept a male until 7 to 10 days in.

"Most dogs" hides a lot of variation, because proestrus alone can last anywhere from 3 days to 3 weeks. If you are breeding, ask your vet about progesterone blood tests: they show when the LH surge and ovulation happen, which pins down both the best mating days and the due date. Once she is mated, the dog due date calculator gives her whelping window.

Looking after a dog in heat

Prevent accidental matings. She can get pregnant on any day of estrus. Walk her on a lead, never leave her alone in the yard, and keep her apart from intact males for the whole heat.
Protect your furniture. Washable bedding, a crate cover or dog diapers help during the bloodiest days.
Watch her 4 to 6 weeks after the heat. This is when pyometra, a womb infection of unspayed dogs, usually appears, mostly in dogs over 5. Drinking and peeing more, lethargy, vomiting, a swollen belly or a pus-like discharge need a vet the same day.
Expect some odd behavior after it. A false pregnancy can start at the end of the cycle, with nesting, mothering toys, swollen mammary glands and even milk. It usually settles within 1 to 3 weeks; ask your vet if it doesn't.

Frequently asked questions

How long does a dog stay in heat?

Most heats last about 2 to 3 weeks from the first sign of swelling or bleeding until she stops accepting males. Proestrus and estrus each average 9 days, but either can be as short as 3 days or as long as 3 weeks.

Can a dog get pregnant when she is not in heat?

No. She can only conceive around ovulation, which happens during estrus. But sperm can survive inside her for about a week, so a mating late in proestrus, before she seems ready, can still lead to puppies.

Do dogs bleed the whole time they are in heat?

Usually not. The discharge is bloodiest during proestrus, the first week or so, then becomes lighter, watery or pink during estrus, which is also when she is most fertile. Lighter bleeding does not mean the heat is over.

Do older female dogs stop going into heat?

No. Dogs don't go through a menopause like people do. Heats may become less obvious with age, but an unspayed senior can still get pregnant, and her risk of a womb infection (pyometra) goes up.

What are the signs that a dog is going into heat?

A swollen vulva and a bloody discharge are the first signs. Many dogs also pee more often or mark with urine, and some become anxious or vocal, or start mounting. Male dogs suddenly taking an interest in her is often the giveaway.

Sources

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