Dog Heat Cycle Calculator — Estrus Phase & Fertile Window

When is your bitch fertile, and how long does her heat actually last? This calculator tracks the canine estrus cycle through the four classic phases — proestrus → estrus → diestrus → anestrus — and predicts the fertile breeding window from the start of bleeding. For exact ovulation timing in a planned breeding, layer in serial progesterone testing as outlined by the AKC canine reproduction guide.

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Day 1 of heat (first bloody discharge)

Cycle length (months between heats)

Enter the first day of bleeding.

Mating-date timing vs progesterone testing: which gives a more accurate fertile window?

Aspect	Mating-date / cycle-day method	Progesterone testing
Ovulation accuracy	± 3-5 days	± 12-24 hours
Cost	Free	$60-150 across 2-4 draws
Best for	Hobby pet pregnancy	Planned breeding, shipped semen, c-section scheduling
Confirms LH surge	No	Yes (when P4 rises above 2 ng/mL)

Canine estrus terminology

Proestrus: Bloody discharge phase, 7-10 days; bitch attracts but does not accept males.
Estrus: Standing heat, 5-10 days; bitch accepts mating. Ovulation occurs days 11-14 of full cycle.
Diestrus: Post-ovulation luteal phase, 60 days; pregnant or pseudopregnant.
Anestrus: Reproductive resting phase between cycles, typically 4-5 months.

Frequently Asked Questions

How long does a dog's heat cycle last?

A full heat cycle (proestrus + estrus) lasts 2–4 weeks. Proestrus (bleeding, non-receptive) is 7–10 days, estrus (fertile) is another 5–10 days. Complete ovarian cycle between heats is typically 6 months.

When is a dog most fertile?

Days 9–14 of the cycle (estrus phase). Ovulation typically occurs around day 11–13 after start of bleeding. For precise breeding, serial progesterone (P4) blood testing pinpoints ovulation.

How often do dogs go into heat?

On average every 6 months, but this varies: small breeds every 4 months, giant breeds every 8–12 months. Heats are typically more regular after the second cycle.

At what age do dogs first go into heat?

Small breeds: 6 months. Medium: 9–12 months. Large/giant: 12–24 months. First heats can be irregular — breeding should wait until the second or third cycle (18+ months) to protect the bitch's health.

How many days is a dog in heat?

Bleeding (proestrus) lasts 7–10 days; receptive fertile phase (estrus) is another 5–10 days. Total visible heat: 2–4 weeks.

Can a dog get pregnant during proestrus?

Unlikely but not impossible. Sperm can survive 5–7 days in the reproductive tract, so a mating on day 7 (late proestrus) can still lead to fertilization around day 11 (ovulation). Separate bitch from males throughout the entire heat.

Sources & References

[1]
Estrous cycle of the dog — Merck Veterinary Manual
[2]
AKC Canine Reproduction — American Kennel Club
[3]
American Veterinary Medical Association — Pet Owner Resources — AVMA

Common mistakes to avoid

Letting the bitch free-roam during proestrus/estrus — accidental matings happen through fences, open doors.
Breeding on first heat — bodies aren’t fully developed. Wait until 2nd or 3rd cycle minimum.
Forgetting the 21-day commitment — full heat cycle requires containment for 3 weeks.

Pro tips

Progesterone testing (at the vet, $50-100) identifies ovulation day ± 24h — best timing for natural or AI breeding.
Log heats in a calendar — typical cycle is every 6-7 months, but varies.
Post-heat, pyometra risk rises — any lethargy + increased drinking + vaginal discharge 4-8 weeks after heat is vet-emergency.