Meddor
An unspayed dog comes into heat every 7 months on average, but each has her own rhythm. Enter the first day of her last heat: the calculator estimates the next one, and learns her rhythm as soon as you give it two dates.
How often?
According to the Merck Veterinary Manual, the interval between two heats ranges from 4 to 13 months, 7 months on average. Heats come all year round, with no season. That’s why your dog’s own rhythm is the most reliable guide: note the date each heat starts.
The first heat comes between 6 and 24 months of age: earlier in small breeds, later in large ones.
How long it lasts
| Phase | What you see | Length |
|---|---|---|
| Proestrus | Bloody discharge, swollen vulva. She attracts males but doesn’t accept them yet. | 7 to 10 days of discharge |
| Estrus | She accepts a male. Discharge may lessen. | 9 days on average, 3 to 21 |
Ovulation usually happens 1 to 3 days after estrus begins. Sperm can survive nine days or more: a mating before the fertile window can still lead to a pregnancy.
What about cats?
Cats are different: during the season, from spring to early autumn, heat comes back every 2 to 3 weeks and lasts 6 to 7 days. A cat ovulates when she’s mated. Her first heat comes between 4 and 12 months. You’ll recognise it by her behaviour: rolling, rubbing, kneading and loud yowling.
Frequently asked questions
Do the app and this calculator give the same date?
Almost. The app averages every interval you’ve logged; this calculator only uses the last two dates. A gap under 3 months or over 18 months looks like a missed or extra date: the app leaves it out and falls back on the 7-month average, the calculator asks you to check the dates. Outside the usual 4 to 13 months according to Merck, the calculator suggests talking to your vet.
My dog is expecting puppies: when will they arrive?
Our pregnancy calculator gives you the due-date window from the mating date.
Her heats are irregular
According to Merck, a dog can differ from the average without being ill, and only a vet can tell whether her cycle is abnormal. Tell them what you’ve noted: the dates will help.
Should I have her spayed?
That’s a decision to make with your vet, who knows your dog and can explain the benefits and risks for her age and breed.
Sources
- Merck Veterinary Manual, “Breeding Management of Bitches”, July 2025: interval between heats, estrus length, sperm survival.
- Merck Veterinary Manual, “Features of the Reproductive Cycle”: puberty, cycle, discharge, ovulation, cats.
- Merck Veterinary Manual, “Management of Reproduction of Cats”, August 2025: behaviour of a cat in heat.