Dog Heat Cycle Calculator
A medium-breed dog cycles about every 6 months, so a heat that began May 1, 2026 points to the next one around November 1, 2026. The fertile window runs November 8 to 15, with ovulation near November 10. Breeding then suggests whelping near January 12, 2027.
| Current cycle day (from last heat) | |
| Current stage | |
| Next heat, average | |
| Next heat, early side | |
| Next heat, late side | |
| Fertile window for next heat | |
| Ovulation day for next heat | |
| Due date if bred on the ovulation day |
Every calculation runs in your browser. Nothing you type is uploaded or stored.
Enter the start date of your dog's last heat and her breed size. The calculator returns the next heat window, the fertile days, and the four stages ahead.
Key takeaways
- Medium breed cycle is about 6 months (roughly 180 days)
- May 1, 2026 heat points to the next near November 1, 2026
- Fertile days are the next heat's day 8 to 15
- Ovulation averages day 10
- Whelping comes 63 days after ovulation
On this page
How the dog heat cycle works
A dog's heat cycle has four stages that repeat about every 6 months in most dogs, with smaller breeds cycling more often and giant breeds cycling less often. The VCA Animal Hospitals: estrous cycles in dogs from VCA lays out the same stages and timings, and the Merck Veterinary Manual: breeding management of bitches covers the breeding side in more detail. The four stages are:
- Proestrus (about 7 to 10 days). Vulva swells, bloody discharge starts. The female is not yet receptive to males. Sperm can survive up to 7 days in the tract from this point on.
- Estrus (about 9 days, range 2 to 24). Discharge thins and turns straw-colored, the female stands to be bred, and ovulation occurs around day 9 to 11 from the start of proestrus.
- Diestrus (about 60 days). The body returns to baseline, or pregnancy continues if conception happened. The female is no longer receptive.
- Anestrus (about 4 to 5 months). The quiet phase. No outward signs. The cycle starts again when the next proestrus begins.
Worked example: medium breed, heat on May 1
Worked example: last heat started May 1, 2026, medium breed
Your medium-breed dog came into heat on May 1, 2026 and you want to know when the next one is and the fertile window for breeding.
- Next heat, average (6 months): November 1, 2026
- Next heat, early side (5 months): October 1, 2026
- Next heat, late side (7 months): December 1, 2026
- Fertile window, average: November 8 to November 15, 2026 (day 8 to 15 from proestrus start)
- Ovulation day, average: November 10, 2026
A tie on November 10, 2026 produces puppies around January 12, 2027 (63 days later). The dog-pregnancy calculator handles the same 58 to 68 day window in detail.
The four stages, day by day
What the four stages look like, day by day
The table below assumes a 6 month cycle and an 8 to 18 day receptive window, the working averages from the VCA guide. Days are counted from the start of proestrus.
| Stage | Days | What you see | Behavior |
|---|---|---|---|
| Proestrus | 1 to 9 | Vulva swells, bloody discharge starts | Not yet receptive, males are highly interested |
| Estrus | 9 to 18 | Discharge thins to straw color | Stands to be bred, flagging, accepts the male |
| Ovulation | 9 to 11 | Discharge thinnest, clear stretch | Peak fertility, breed every other day for best coverage |
| Diestrus | 18 to 78 | Discharge stops, vulva shrinks | No longer receptive, body returns to baseline |
| Anestrus | 78 to 180+ | No visible signs | Normal, no hormone-driven behavior |
The numbers are averages. A real cycle can run 2 to 24 days for estrus, per VCA, and the interval between cycles can range from 4 to 12 months depending on breed size. The calculator returns a window that covers about 80 percent of dogs.
Cycle length by breed size
Cycle length by breed size
VCA notes that small breeds tend to cycle every 4 to 6 months, medium breeds 5 to 7 months, large breeds 6 to 8 months, and giant breeds 8 to 12 months. The calculator uses these ranges when you pick a size.
| Breed size | Average cycle | Range | First heat |
|---|---|---|---|
| Small (under 20 lb) | 5 months | 4 to 6 months | 4 to 6 months |
| Medium (20 to 50 lb) | 6 months | 5 to 7 months | 6 to 9 months |
| Large (50 to 90 lb) | 7 months | 6 to 8 months | 8 to 14 months |
| Giant (over 90 lb) | 9 months | 8 to 12 months | 12 to 24 months |
The stages of the cycle are the same length for every breed, what changes with size is the quiet phase (anestrus) between heats.
Reading the signs in real time
Reading the signs in real time
The first sign most owners see is a swollen vulva, followed by bloody discharge. Many dogs lick the area more than usual, and many urine-mark in the house or on walks. The blood is bright red at first and thins to pink or straw color as estrus begins. The most reliable sign of estrus itself is behavior: a female in estrus flags her rear to the side when touched, holds her tail to the side, and stands to be mounted. Males will detect a female in heat from a long distance and will mark, whine, or try to reach her.
For breeding, the gold-standard test is a blood progesterone series. The first sample is taken at day 7 from the start of proestrus, and the rise in progesterone pinpoints ovulation within a day or two. The VCA Animal Hospitals: estrus and mating in dogs page from VCA covers the same timing in plain English.
How to use this calculator
How to use this calculator
- Enter the date your dog came into heat (the first day you saw swelling or bloody discharge).
- Pick her breed size. The default is a 6 month cycle, which fits most medium breeds.
- Read the next heat window and the fertile days. The fertile window is the time to breed if you are planning a litter.
- If you do breed, the due date is the breeding date plus 63 days. Our dog pregnancy calculator handles the full 58 to 68 day window.
- If you are not breeding, keep the female away from intact males for the full proestrus and estrus period, and watch for swelling, discharge or flagging as a reminder of the next cycle.
The numbers are working averages from VCA and the Merck Veterinary Manual. Every dog is a little different. A progesterone test is the only way to confirm ovulation, and a vet examination is the only way to confirm pregnancy.
Frequently asked questions
How often do female dogs go into heat?
Most dogs come into heat about every 6 months. Small breeds can cycle 3 times a year, and giant breeds can cycle only once a year, according to the VCA Animal Hospitals guide to estrous cycles. The interval is regular in a mature dog but the first year or two can be uneven.
When is a dog most fertile?
A dog ovulates about 9 to 11 days after the first signs of proestrus (the bleeding and swelling), and is receptive to the male from about day 7 to day 18 of the cycle. Sperm can survive up to a week inside the female, so a tie that happens 3 to 5 days before ovulation can still produce a pregnancy.
How long does each heat last?
The heat itself (proestrus plus estrus) usually lasts 2 to 3 weeks. Proestrus runs about 7 to 10 days, with bloody discharge and a swollen vulva. Estrus runs about 9 days on average, with the discharge thinning to a straw color and the female standing to be bred. The full cycle from proestrus through anestrus runs about 6 months.
What are the four stages of the dog heat cycle?
Proestrus (about 7 to 10 days, swelling and bloody discharge, female is not yet receptive), estrus (about 9 days, female is receptive, ovulation occurs), diestrus (about 60 days, the body returns to baseline or pregnancy continues), and anestrus (about 4 to 5 months, the quiet phase before the next cycle).
When does a dog have her first heat?
Around 6 months on average, but it ranges from 4 months in small breeds up to 18 to 24 months in giant breeds. VCA notes that small breeds tend to have their first cycle earlier and giant breeds can take up to two years. A first heat can be irregular and shorter than later cycles.
How do I tell when my dog is in heat?
The earliest sign is a swollen vulva, followed by bloody vaginal discharge. Many dogs urinate more often and flag their position to signal males. Behavior can shift, some females become clingy, others irritable. Male dogs can detect a female in heat from a long distance.
How accurate is the next-heat estimate?
The 6 month default is an average. Real dogs can run 4 to 8 months between cycles, with small breeds more often and giant breeds less often. The calculator returns a range that covers about 80 percent of cycles. A vet can run a progesterone test to confirm the day of ovulation if you are breeding.
Can a dog get pregnant on the first heat?
Yes, dogs can become pregnant on their first cycle. VCA recommends spaying before the first heat to avoid an accidental litter, and notes that breeding a dog on her first heat is not recommended because she is still growing.
When in the cycle can a dog get pregnant?
Anywhere from day 7 to day 18 of the cycle, with peak fertility around day 10 to 14. Sperm live up to 7 days in the female tract, so an early tie can still catch late ovulation. VCA notes a tie is not required for pregnancy, the male and female do not have to 'lock' for a litter to happen.
How long is a dog pregnant?
About 63 days from ovulation, or 58 to 68 days from the first breeding. The VCA estrus page links to the 63 day average and our dog pregnancy calculator handles both methods. The variability is the same as the heat cycle variability, the calendar can shift by several days.
Do small and large dogs have different cycle lengths?
Yes. Small breeds average 4 to 6 months between heats, medium breeds 5 to 7 months, large breeds 6 to 8 months, and giant breeds 8 to 12 months. The stages of the cycle (proestrus, estrus, diestrus, anestrus) are the same length for every breed, but the time between cycles grows with the dog's size.
Should I let my dog have a litter before spaying?
VCA says there are no valid reasons to let a dog have a litter before spay surgery. Spaying at any age increases lifespan and reduces the risk of mammary cancer and a serious uterine infection called pyometra. Some large breeds may benefit medically from delaying spay until after the first heat, so talk to your vet about timing.
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