Dog Heat Cycle Calculator
Predict when your dog's next heat will start, which days she is most fertile, and when her following cycles are due — based on her size, age, and last heat date.
Your dog's heat cycle forecast
Inputs changed — click Calculate to update these results.
Next heat most likely starts
Time until next heat
Current stage (today)
Heats per year
- Proestrus
- Estrus (fertile)
- Diestrus
- Anestrus (rest)
Next heat timeline
| Milestone | Heat day | Date |
|---|
Upcoming heat cycles
| Cycle | Most likely start | Possible range |
|---|
Notes for your dog
Overview
The Dog Heat Cycle Calculator forecasts when your female dog is likely to come into heat next, which days of that heat she is most fertile, and when her following cycles should arrive. Enter the date her last heat started, her size, and her age, and the tool builds a dated timeline you can put straight onto a calendar.
It's built for owners of intact females who want to prevent an accidental litter, plan boarding, daycare, grooming or travel around her season, and book a spay at the right time — and for hobby breeders who need to know when to start progesterone testing.
Heat cycles vary a lot by body size: toy breeds can cycle every four to five months, while giant breeds may go close to a year or more between heats. Young and senior dogs are also less predictable. Instead of promising a single rigid date, the calculator uses size-based veterinary ranges, adjusts for age, and shows the realistic window in which her next heat is likely to begin.
How It Works
- Enter the date her last heat started. Use the first day you noticed a swollen vulva or bloody discharge — the start of the heat, not the end. The tool needs at least one recorded heat; it can't predict a puppy's first heat.
- Select her size. Choose the category that matches her adult weight. Size sets the typical number of months between heats.
- Enter her current age in years. Decimals are fine (1.5 = 18 months). Age widens the forecast for young dogs, whose early cycles are irregular, and lengthens it for seniors.
- Add her known cycle interval (optional). If you've tracked two or more heats, enter the average number of months between them. Her own history overrides the size average and tightens the forecast.
- Click Calculate. You'll see the predicted start date and window, her stage today, the fertile days of her next heat (or of her current heat, if she's in heat now), and the next three cycles. Click Reset to clear everything and start again.
Formula Explanation
Every date is counted from the first day of her last heat (Day 1). Months are converted to days using the average month length of 30.44 days.
Step 1 — Cycle interval
If you enter her known interval, that value is used with a possible range of ±15% of the interval, kept between 2 and 6 weeks. Otherwise the size baseline below applies:
| Size | Typical interval | Normal range |
|---|---|---|
| Toy | 5 months | 4–6.5 months |
| Small | 6 months | 5–7.5 months |
| Medium | 6.5 months | 5.5–8 months |
| Large | 7 months | 6–8.5 months |
| Giant | 11 months | 8–18 months |
Step 2 — Age adjustment
Step 3 — Next heat and its window
If today is already past the end of the possible range, the calculator assumes a heat went unrecorded and projects forward to the next cycle whose range hasn't closed. If she's in heat right now (day 1–21), the timeline shows her current heat instead.
Step 4 — Later cycles
Each cycle varies independently, so uncertainty grows with the square root of the number of cycles projected — the third forecast is wider than the first.
Step 5 — Stages inside a cycle
The fertile window opens on the last day of proestrus because sperm can survive several days inside the female before ovulation.
Worked example
Practical Benefits
Prevent accidental litters
Know weeks in advance when to switch to on-leash walks, skip dog parks, and secure your yard from roaming males.
Time breeding accurately
See when to start progesterone testing and which days to have the stud dog on standby, so you don't miss a narrow fertile window.
Schedule a spay safely
Most vets prefer to operate between heats. The timeline shows when that window typically opens so you can book ahead.
Plan boarding, travel and events
Many daycares, kennels, groomers and shows turn away dogs in heat. Plan trips and bookings around her season.
Spot irregular cycles early
A heat that's far outside her normal window can signal a health issue. Tracking against a forecast makes changes obvious.
Be ready at home
Have washable bedding, season pants and cleaning supplies ready before the first spotting, not after.
Frequently Asked Questions
Most dogs come into heat about twice a year, roughly every six to seven months, but size makes a big difference. Toy and small breeds may cycle every four to six months, while giant breeds such as Great Danes or Mastiffs often cycle only every eight to eighteen months. A first heat usually appears between 6 and 15 months of age in small dogs and can be delayed until 18–24 months in giant breeds. Each heat lasts about two to three weeks.
For most dogs the fertile window falls around days 9 to 16 of the heat, counting the first day of bleeding as Day 1, with peak fertility around days 11 to 14. Discharge often lightens to pink or straw-coloured and she may "flag" her tail. However, some dogs ovulate as early as day 5 or as late as day 25, so day counts are only a guide. If you're breeding, progesterone blood tests from your vet pinpoint ovulation. If you're preventing pregnancy, keep her away from intact males for the entire heat — at least three weeks.
Individual variation is normal, and several things shift timing: young dogs often have irregular or "split" heats that start, stop and restart; some dogs have "silent" heats with almost no visible signs; stress, illness, travel, significant weight change and living with other females can all move a cycle; and conditions such as hypothyroidism can delay heats. If she goes more than 12 months without a heat (18 months for giant breeds), or cycles more often than every four months, have your vet check her. Each time a heat actually starts, re-enter that date for a sharper forecast.
No — dogs don't go through menopause. Intact females keep cycling for life, although the gap between heats usually lengthens and the signs become lighter, so a senior can still become pregnant. The risk of pyometra, a serious uterine infection, also rises with each cycle. Watch older females closely for two to eight weeks after every heat for lethargy, increased thirst, poor appetite or vaginal discharge, and contact your vet immediately if you see them. Discuss spaying with your vet if you don't plan to breed.
Disclaimer
This calculator provides estimates based on typical canine reproductive patterns and is intended for general informational purposes only. It is not veterinary advice and cannot confirm ovulation, pregnancy, or reproductive health. Individual dogs vary widely. Do not rely on these dates alone to prevent pregnancy or to time breeding; consult a licensed veterinarian for diagnosis, progesterone testing, spay timing, and any concerns about your dog's health.
