Dog heat cycle calculator: when is her next heat?
Enter the day your dog’s last heat started and her size. You get the window in which her next heat is most likely to start, the normal range around it and how long each stage usually lasts. It’s an estimate from veterinary sources: every dog’s cycle is her own.
Estimate her next heat
Estimate
Next heat estimate
Last heat started on April 14, 2026
If she keeps her own rhythm, her next heat may start
October 19, 2026 – November 8, 2026
Her last two heats started 198 days apart (about 6.5 months). The window is that gap again, ±10 days. Some dogs repeat their gap to within 5–10 days; in others it varies a lot (Concannon, 2011). If she isn’t regular, go by the window for her size below.
- Usual window for her size
October 14, 2026 – November 14, 2026
6 to 7 months after her last heat began: the most common interval (Concannon). Most dogs come into heat twice a year (VCA).
- Normal range: 4 to 13 months
August 14, 2026 – May 14, 2027
The Merck Veterinary Manual gives 4–13 months between heats as normal, 7 months on average. If her heat comes earlier or later, ask your vet.
- Once her heat starts
The first stage (proestrus) lasts about 9 days, anywhere from 3 days to 3 weeks; the second (estrus) also about 9 days, from 3 days to 3 weeks (Merck). Together they usually take 2–3 weeks (Concannon).
In young dogs the first cycles are often irregular: it can take up to two years for them to settle (VCA).
Questions about her cycle, breeding or spaying: ask your vet.
This is an estimate, not a medical prediction. Everything is calculated in your browser; nothing is sent or stored.
A reminder before her next heat
In Pettify, a reminder fires once or repeats daily, weekly, monthly or yearly, and you can mark it done or snooze it right from the notification. Note the start of each heat in your dog’s diary, with photos if you like.
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How the calculator works
The window for her size
The calculator adds months to the day her last heat started. Most dogs come into heat twice a year (VCA Animal Hospitals), and the time from one heat to the next is commonly 6 to 7 months (Concannon, 2011). For a medium or large dog, the window runs from 6 to 7 months after the last start.
Size moves the ends. Small breeds may cycle up to three times a year, about every 4 months, so their window opens at 4 months. Giant breeds may cycle only once a year, so theirs stays open until 12 months.
Her own rhythm
If you also enter the start of the heat before, the calculator measures the gap between the two and adds it once more. Concannon notes that within one dog the gap ranges from highly variable to regular, perhaps within 5–10 days. That’s why the window around her rhythm is ±10 days and only holds for a regular dog.
A gap shorter than 4 months or longer than 13 isn’t used: it falls outside the normal range, and the calculator falls back on the window for her size.
What it can’t tell you
It’s a calendar estimate, nothing more. It can’t tell which stage she is in today or anything about her health. Only your vet can examine her and answer that.
Stages of the heat cycle and how long they last
The four stages as the Merck Veterinary Manual describes them. The lengths are typical ranges; any one dog can differ.
| Stage | Usual length | What happens |
|---|---|---|
| Proestrus | About 9 days (3 days to 3 weeks) | The heat begins: a bloody discharge, a mildly swollen vulva. Males are attracted, but she won’t accept them yet. |
| Estrus | About 9 days (3 days to 3 weeks) | She accepts a male. The discharge may lessen; the swelling is at its greatest. |
| Diestrus | 2–3 months if she isn’t pregnant | She no longer accepts males and attracts them less. Discharge and swelling fade. |
| Anestrus | 1–6 months | The resting stage until the next heat: she neither attracts nor accepts males. |
How often do dogs come into heat?
Most dogs come into heat twice a year, but the interval differs between breeds and from dog to dog (VCA). The Merck Veterinary Manual gives 4 to 13 months between heats as normal, with 7 months on average.
The first heat comes at about 6 months on average; smaller breeds tend to start earlier, and large and giant breeds may not come into heat until 18 months to 2 years (VCA). Merck’s range for puberty is 6 to 24 months. Early cycles are often irregular, and it can take up to two years for them to settle.
Keep a record of her heats
Each date you write down makes the next estimate better and gives your vet a clear history. Our printable heat log has room for every cycle and a day-by-day page for the current one.
Frequently asked questions
How long is a dog in heat?
The heat, proestrus and estrus together, usually lasts 2–3 weeks (Concannon, 2011). Merck gives each stage about 9 days on average, from 3 days to 3 weeks.
How many times a year does a dog go into heat?
Usually twice. Small breeds may cycle three times a year, giant breeds only once (VCA).
Can the calculator tell me her fertile days?
No. It estimates when the next heat may start, from dates alone. Ask your vet about breeding, pregnancy or spaying.
Does the calculator save or send what I enter?
No. It runs in your browser, nothing is sent anywhere, and nothing is stored: reload the page and it starts over. The calendar file is made on your device.
Sources
- Merck Veterinary Manual. Davidson A.P. Breeding Management of Bitches. 2025. merckvetmanual.com (opens in a new tab) (accessed )
- Merck Veterinary Manual. Table: Features of the Reproductive Cycle. 2025. merckvetmanual.com (opens in a new tab) (accessed )
- Animal Reproduction Science. Concannon P.W. Reproductive cycles of the domestic bitch. 2011. doi.org (opens in a new tab) (accessed )
- VCA Animal Hospitals. Weir M., Llera R., Yuill C. Estrus Cycles in Dogs. 2025. vcahospitals.com (opens in a new tab) (accessed )
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