🐾 Dog Heat Cycle Calculator
Enter your dog's last heat date and size to estimate when her next heat will begin, her fertile window, and each stage of her cycle.
How the Dog Heat Cycle Works
Female dogs typically go into heat (estrus) every 6–8 months, though this varies by breed and individual dog. Small breeds may cycle more frequently — sometimes every 4–5 months. Giant breeds often cycle only once a year. Most dogs settle into their individual pattern by their second or third heat.
The complete canine reproductive cycle is called the estrous cycle and consists of four stages. Understanding each stage helps you anticipate your dog's behavior, plan around heat, and know when fertility is highest.
The Four Stages of the Dog Estrous Cycle
Proestrus (Days 1–9, average): The first visible stage of heat. Estrogen rises, the vulva swells, and a bloody vaginal discharge appears. Most dogs are not yet receptive to mating during this phase despite attracting male attention. Duration typically ranges from 3 to 17 days.
Estrus (Days 9–14, average): The fertile window. Estrogen drops, progesterone rises, and ovulation occurs. The discharge often lightens in color. The dog becomes receptive to mating. This is the stage this calculator estimates as the "fertile window." Duration is typically 5–13 days.
Diestrus (Days 14–75, average): The post-fertile phase. Whether pregnant or not, progesterone remains elevated for approximately 2 months. Pregnant dogs will have puppies during this phase. Non-pregnant dogs go through a "phantom pregnancy" hormonal state — some show nesting behavior or mammary development regardless.
Anestrus: The resting phase between cycles. Reproductive hormones are at baseline. No outward signs. This phase lasts until the next proestrus begins — typically 4–6 months for most dogs.
Signs Your Dog Is in Heat
- Swollen vulva
- Bloody or straw-colored vaginal discharge
- Increased urination (scent-marking behavior)
- Attracting male dogs
- Tail flagging (holding tail to the side when touched near the rump)
- Restlessness or behavior changes
- Increased affection toward people, or increased independence
Managing Heat If You're Not Breeding
If you're not planning to breed your dog, the best long-term solution is spaying. Dogs who are spayed before their first heat have significantly lower risk of mammary cancer and eliminate the risk of pyometra, a life-threatening uterine infection that affects roughly 1 in 4 intact females by age 10.
If your dog is not yet spayed, managing heat involves keeping her away from intact males (even brief contact can result in pregnancy), using dog diapers for discharge management indoors, and avoiding dog parks or off-leash areas during the estrus phase.
When to Track the Cycle With Your Vet
If you are planning to breed your dog, calendar-based prediction alone is not sufficient for accurate timing. Vaginal cytology (microscope evaluation of vaginal cells) and progesterone blood tests provide far more precise fertility timing. These tests are typically done every 1–2 days during the heat cycle and can identify the optimal breeding window within 24–48 hours.
This calculator is best used as a planning tool — to anticipate when heat is likely to occur — rather than a substitute for veterinary-guided timing if breeding is the goal.
If mating occurs, use our Dog Pregnancy Calculator to estimate due date and track each week of gestation.
Frequently Asked Questions
How often do dogs go into heat?
Most dogs go into heat every 6–8 months, but this varies by size and individual dog. Small breeds may cycle every 4–5 months. Giant breeds sometimes cycle only once a year.
How long does a dog's heat cycle last?
The full visible heat period (proestrus + estrus) typically lasts 2–4 weeks. Proestrus averages 9 days and estrus (the fertile window) averages 5–9 days, though both can vary significantly between individual dogs.
When is a dog most fertile during her heat cycle?
Dogs are most fertile during the estrus phase, which typically begins around day 9–10 of the heat cycle and lasts 5–9 days. For accurate breeding timing, veterinary progesterone testing is more reliable than calendar prediction alone.
Can it project my dog's next heat as a calendar?
Yes. Enter your dog's last heat start date and it projects the next expected heat, the proestrus and fertile estrus windows within it, and roughly when that heat should end. Individual dogs vary from these averages, so treat it as a planning estimate — and the more past cycle dates you have to compare against, the better you'll know how closely your own dog follows it.