Dog Heat Cycle Calculator: Fertile Days, Breeding Dates and Next Heat
Track a female dog’s heat from the first day of bleeding: proestrus, estrus, diestrus and anestrus dates, the estimated LH surge, ovulation and fertile window, breeding days for fresh or frozen semen, high-risk days if you want to avoid a litter, the due date if she conceives, the pyometra watch window and when her next heat is due by breed size.
How it works
Day 1 = first bleeding · LH surge ≈ day 10 (proestrus averages 9 days), or the day progesterone first reaches about 2 ng/mL · Ovulation ≈ LH + 2 days · Fertile window ≈ LH + 3 to LH + 7 · Breeding: fresh semen LH + 4 and LH + 6, frozen semen LH + 6 and LH + 7 · Estrus ≈ 9 days from the LH surge · Due date ≈ LH + 65 days (64–66) · Next heat = start + breed interval (4–13 months)
A female dog’s cycle has four phases. In proestrus (average 9 days, range about 3–21) estrogen rises, she bleeds and swells, and males are interested but she refuses them. Near the end of proestrus a surge of luteinising hormone (LH) triggers ovulation about 2 days later. Dog eggs are released immature and need about 2 more days before they can be fertilised, then survive 2–3 days, so the fertile window is roughly 3 to 7 days after the LH surge. Estrus, when she stands for males, also averages 9 days. Diestrus follows for about 2 months whether or not she is pregnant, then anestrus, the resting phase, lasts until the next heat. The calculator places the LH surge on day 10, the average, and builds every date from it. Real dogs vary a lot: some ovulate on day 5, others after day 20. For breeding, progesterone blood tests find the LH surge (progesterone first rising to about 2 ng/mL); enter that day under More options and the fertile window, breeding days and due date are recalculated. IDEXX recommends breeding on days 4 and 6 after the LH surge with fresh semen and days 6 and 7 with chilled or frozen semen. Puppies are due 64–66 days after the LH surge. If you want to avoid a litter, treat the whole heat as risky: sperm can survive up to about a week, and some dogs are receptive while still bleeding. The next heat is projected from her own interval or the typical interval for her size: about twice a year on average, up to three times a year in small breeds and about once a year in giant breeds and Basenjis. After every heat an unspayed dog is at risk of pyometra, most often 4–6 weeks after the heat, and occasionally anywhere from 2 to 16 weeks.
Worked example
A medium-sized dog starts bleeding on Mar 1, 2026. Without progesterone testing the LH surge is estimated on day 10 (Mar 10, 2026) and ovulation on day 12 (Mar 12, 2026). The fertile window is days 13–17, Mar 13 to Mar 17, 2026, and the highest-risk days for an unwanted pregnancy are days 6–17, Mar 6 to Mar 17, 2026; keep her away from males until at least Mar 28, 2026. For a planned natural mating, the best days are day 14 (Sat, Mar 14, 2026) and day 16 (Mon, Mar 16, 2026). Her heat should end around Mar 18, 2026, and the next one is expected between Sep 1 and Oct 1, 2026. If she conceives, puppies are due about May 14, 2026. Watch for pyometra from Apr 1 to Jul 8, 2026, most often Apr 15 to Apr 29, 2026.
Sources: UC Davis School of Veterinary Medicine: Canine estrous cycle and breeding management · IDEXX: Simplifying the canine reproductive cycle using progesterone · VCA Animal Hospitals: Estrus cycles in dogs · VCA Animal Hospitals: Pyometra in dogs
Frequently asked questions
How long is a dog in heat?
Usually 2 to 3 weeks: proestrus (bleeding, about 9 days) and estrus (receptive, about 9 days). Both phases can last from about 3 days to 3 weeks, so some dogs are in heat for a month.
When is a dog most fertile?
About 3 to 7 days after the LH surge, which on average falls around days 13–17 after bleeding starts. The calendar is only a guide; progesterone tests show the real timing.
Can a dog get pregnant while still bleeding?
Yes. Some dogs ovulate while the discharge is still red, and sperm can survive up to about a week, so a mating early in the heat can still lead to puppies. Treat the whole heat as fertile.
How often do dogs go into heat?
Most dogs cycle about every 6–7 months (twice a year). Small breeds can cycle up to three times a year, while giant breeds and Basenjis often cycle only once a year. Cycles can be irregular for the first 2 years.
When should I worry about pyometra after a heat?
Pyometra usually appears 4–6 weeks after a heat but can occur from 2 to 16 weeks after. Increased thirst and urination, lethargy, vomiting, a swollen belly or discharge from the vulva in an unspayed dog need a vet the same day.
How is the due date worked out?
Puppies are born 64–66 days after the LH surge, which is more precise than counting from mating (about 63 days, but anywhere from 58 to 68). With progesterone testing the due date is accurate to a couple of days.
Related calculators
General veterinary guidance (UC Davis, VCA, IDEXX reproduction references). Every dog differs and calendar dates are estimates: for breeding use progesterone testing with your veterinarian, and see a vet at once for any sign of pyometra.