🌡️ Reproductive Health

Dog Estrus Calculator

Predict your dog’s next heat cycle, fertile window, and optimal breeding dates using veterinary reproductive science.

🐾 Introduction

The canine estrous cycle is a precisely orchestrated hormonal sequence that governs a female dog’s reproductive capacity. Understanding when your dog will come into heat — and identifying the fertile window within that cycle — is critical for both planned breeding and responsible pet ownership.

This calculator uses breed-size–adjusted interestrus intervals derived from longitudinal studies in canine reproductive physiology. It estimates the next heat onset, the peak fertile window (ovulation ± 2 days), and — if breeding occurred — an estimated whelping date.

📚 Background: The Canine Estrous Cycle

The Four Phases

  • Proestrus (avg 9 days; range 3–17): Bloody vaginal discharge begins. Males are attracted but the female is not yet receptive. Rising estrogen drives uterine and vulvar changes.
  • Estrus (avg 9 days; range 3–21): Female accepts mating. The LH (luteinizing hormone) surge triggers ovulation approximately 24–48 hours after its peak. Eggs require a further 48–72 hours to mature before they are fertilizable.
  • Diestrus (56–90 days): Progesterone dominates. If pregnant, this period ends at parturition (~63 days post-ovulation). Non-pregnant females undergo pseudopregnancy risk.
  • Anestrus (variable, ~4–5 months): Reproductive quiescence. Duration determines overall cycle length and varies significantly by breed size.

Breed-Size Interestrus Intervals

The time from the start of one heat to the start of the next (the interestrus interval) varies by body size:

  • Small breeds (<10 kg): 4–6 months (avg 150–180 days)
  • Medium breeds (10–25 kg): 5–7 months (avg 180–210 days)
  • Large breeds (25–45 kg): 6–8 months (avg 195–240 days)
  • Giant breeds (>45 kg): 7–12 months (avg 240–365 days)

Fertile Window

The most fertile period is Days 11–13 of the estrous cycle (counting from Day 1 = first day of proestrus bleeding), corresponding to 2–4 days post-LH surge. However, canine sperm can survive 4–6 days in the reproductive tract, and eggs remain viable for 48–72 hours post-ovulation. Vaginal cytology and progesterone assays provide the most accurate timing in clinical practice.

📋 How to Use This Calculator

  1. Enter the first day of your dog’s last heat (Day 1 = first day bloody discharge appeared).
  2. Select your dog’s breed size to apply the appropriate interestrus interval range.
  3. If you know your dog’s actual cycle length from previous cycles, enter it for a more precise prediction.
  4. If she was bred during the last heat, check the box to receive an estimated whelping date.
  5. Click Calculate to see predicted dates, fertile window, and current cycle phase.

🌡️ Estrus Calculator





⚠️

Accuracy note: Calendar-based prediction is approximate. A dog’s individual cycle can vary by 2–6 weeks from the average. For breeding, progesterone blood tests (target 5–8 ng/mL at ovulation) and vaginal cytology provide far more accurate ovulation timing. Always work with a veterinarian for planned breeding programs.

📖 References & Methodology

Interestrus intervals and cycle phase durations sourced from:

  1. Concannon PW. Physiology and endocrinology of canine pregnancy. Current Therapy in Theriogenology. 1986.
  2. Johnston SD, Root Kustritz MV, Olson PNS. Canine and Feline Theriogenology. WB Saunders, 2001.
  3. Feldman EC, Nelson RW. Canine and Feline Endocrinology and Reproduction, 3rd ed. Saunders, 2004.
  4. Verstegen-Onclin K, Verstegen J. Endocrinology of pregnancy in the dog. Theriogenology. 2008;70(3):291-299.