Dog Heat Cycle Calculator
This dog heat cycle calculator predicts your dog's next heat date, estimated peak fertility window, and estimated end of her current heat. Enter the date her last heat cycle started to get instant estimates.
Quick Answer
Dogs go into heat approximately every 6 months on average, with each heat cycle lasting 18–21 days. Peak fertility typically falls around days 9–15 of the cycle. Enter the date your dog's last heat started above to estimate her next heat, fertile window, and end of the current cycle.
Dog Heat Cycle Calculator
Free tool — no signup required
Enter the first day you noticed signs of heat (swelling, discharge, or increased male attention).
What Is the Dog Heat Cycle?
The dog heat cycle — technically the estrous cycle — is the recurring reproductive cycle of an intact (unspayed) female dog. It has four biological stages: proestrus (swelling and bleeding begin, but the female isn't yet receptive to mating), estrus (the fertile phase, when she is receptive and ovulation occurs), diestrus (the cycle winds down, whether or not pregnancy occurred), and anestrus (a resting phase between cycles). In everyday use, “heat” usually refers to the visible proestrus-plus-estrus window — the roughly three-week period with discharge, swelling, and behavioral changes.
Unlike cats, which are induced ovulators that can cycle repeatedly if not mated, dogs are spontaneous ovulators — ovulation happens on its own schedule regardless of mating, and after each heat a dog enters a long resting period (anestrus) before the next cycle begins.
How Long Does Dog Heat Last?
Dog heat lasts an average of 18 to 21 days, though the full range spans roughly 2 to 4 weeks depending on the individual dog. The cycle typically breaks down like this:
- Days 1–9 (proestrus): Vulvar swelling and bright red discharge begin. Males are attracted, but the female is not yet receptive to mating.
- Days 9–15 (estrus, peak fertility): Discharge often lightens to a straw or pink color. The female becomes behaviorally receptive and ovulation typically occurs during this window — this is the fertile period for breeding.
- Days 15–21 (late estrus/early diestrus): Signs gradually taper off. Swelling reduces and discharge stops as the cycle ends.
Because timing varies by individual, the days above are useful landmarks rather than guarantees — some dogs ovulate as early as day 5 or as late as day 20. For precise breeding timing, veterinary progesterone testing is the standard method.
How Often Do Dogs Go Into Heat?
Most dogs cycle into heat approximately every 6 months, but this interval varies significantly by breed, size, and the individual dog. Some dogs cycle as often as every 4 months; others go a full year between heats. Cycle regularity also tends to become more predictable after a dog's first one or two cycles — early cycles are often irregular in young dogs.
A dog's first heat (puberty) typically occurs between 6 and 12 months of age, though giant breeds can take up to 18–24 months to cycle for the first time. There is no biological menopause in dogs — an intact female will continue to cycle throughout her life, though fertility and litter size typically decline with age.
Signs Your Dog Is in Heat
- Vulvar swelling — often the earliest and most visible sign.
- Discharge — starting bright red and often lightening to pink or straw-colored as the cycle progresses.
- Increased urination and marking — releasing pheromones that signal her status to nearby males.
- Male attention — intact males may be drawn from surprising distances once a female enters proestrus.
- Behavioral changes — restlessness, clinginess, irritability, or, less commonly, mounting behavior.
- “Flagging” — holding the tail to the side when a male approaches or when touched near the tail base, a classic sign of estrus receptivity.
Heat Cycle Frequency by Breed Size
Breed size is one of the strongest predictors of heat cycle frequency — smaller dogs generally cycle more often than larger ones.
| Breed Size | Typical Cycle Frequency | Examples |
|---|---|---|
| Small breeds | Every 4–5 months | Chihuahua, Yorkshire Terrier, Pomeranian |
| Medium/large breeds | Every 6–8 months | Labrador Retriever, German Shepherd, Golden Retriever |
| Giant breeds | Once per year (or less) | Great Dane, Irish Wolfhound, Saint Bernard |
Planning a Breeding? Use Our Gestation Calculator Next
Once you know the mating date — ideally confirmed near peak fertility, around days 9–15 of the cycle — use our dog gestation calculator to calculate the expected whelping date, track week-by-week milestones, and prepare for puppies.
Frequently Asked Questions
What is the dog heat cycle?
The dog heat cycle (estrous cycle) is the recurring reproductive cycle of an unspayed female dog. It has four stages — proestrus, estrus, diestrus, and anestrus — but most owners use 'heat' to mean the visible bleeding and swelling phase (proestrus and estrus combined), which lasts about 18–21 days. During this window a female is attractive to males and, during the estrus portion specifically, fertile.
How long does dog heat last?
Dog heat lasts approximately 18 to 21 days on average, though it can range from 2 to 4 weeks in some individuals. The visible bleeding typically decreases and changes color (from bright red to straw-colored) partway through, which coincides with the fertile window, before tapering off entirely by the end of the cycle.
How often do dogs go into heat?
Most dogs go into heat about every 6 months on average, but this varies meaningfully by breed and size. Small breeds often cycle more frequently — every 4 to 5 months — while large breeds may go 6 to 8 months between cycles, and giant breeds sometimes cycle only once a year. Individual variation is common even within the same breed.
What are the signs my dog is in heat?
Common signs include vulvar swelling, bloody or straw-colored vaginal discharge, increased urination and scent-marking, unusual attentiveness from male dogs, behavioral changes (restlessness, clinginess, or irritability), and 'flagging' — holding the tail to the side when a male approaches, which typically signals she's in the fertile part of the cycle.
When is a dog most fertile during heat?
Peak fertility typically falls around days 9 through 15 of the heat cycle, though this varies by individual — some dogs ovulate earlier or later. This is also roughly when discharge often lightens in color and a female becomes behaviorally receptive to mating. For breeding purposes, progesterone testing is the only reliable way to pinpoint ovulation precisely.
Can I use this calculator to predict my dog's next heat?
Yes — enter the date your dog's last heat cycle started, and the calculator estimates her next heat using the average 6-month interval, along with the estimated peak fertility window and end of the current heat. Because cycle length varies by breed, individual, and can shift over a dog's lifetime, treat the result as a planning estimate rather than an exact date.
Sources & References
- American Kennel Club — Canine reproductive cycle guidelines
- Merck Veterinary Manual — Canine estrous cycle
- Cornell University College of Veterinary Medicine — Canine reproduction
Kanika
Researched and reviewed by Kanika, founder of GestationCalc. All gestation data is verified against primary veterinary and agricultural sources before publication.
Last reviewed: September 8, 2026