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Lambing Percentage Calculator: Born, Weaned & Loss Rates
“Lambing percentage” can name different rates. Enter one age-defined, reconciled cohort and this calculator reports ewes lambing, lambs born, lambs born alive, lambs weaned, and optional weight efficiency with every denominator shown.
Calculate your flock rates
Use one age-class cohort at a time. Copied, printed, and downloaded results keep the formulas and any review warnings with the inputs.
Your named rates appear here
Each result will include its input, denominator, formula, and one boundary to keep with copied, printed, or downloaded output.
Formula key: the words matter as much as the rate
E is eligible ewes exposed; L_E is those ewes that lambed; P_E and F_E are optional scan counts; B_E and A_E are total born and born alive; W_B is the reconciled birth-cohort weaned count. LW_B is its total weaning weight, and EW is the average liveweight of the ewes in L_E.
- Ewes lambing per 100 ewes exposed
- L_E ÷ E × 100
- This is a lambing rate, not a conception rate.
- Scanned-pregnant ewes per 100 exposed
- P_E ÷ E × 100
- Use only when the scan count belongs to the same eligible ewe cohort.
- Fetuses scanned per 100 ewes exposed
- F_E ÷ E × 100
- This is a count per 100 ewes, not a probability.
- Lambs born per 100 ewes lambing
- B_E ÷ L_E × 100
- Ontario calls this lambing percentage. Always keep the denominator in the label.
- Lambs born per 100 ewes exposed
- B_E ÷ E × 100
- B_E includes born-alive, stillborn, and mummified or other recorded birth outcomes for the cohort.
- Lambs born alive per 100 ewes exposed
- A_E ÷ E × 100
- This is separate from total born.
- Birth-cohort lambs weaned per 100 ewes exposed
- W_B ÷ E × 100
- Fostered-in lambs from another birth cohort stay out of W_B.
- Birth-cohort lambs weaned per 100 ewes lambing
- W_B ÷ L_E × 100
- This uses ewes that lambed, not all ewes exposed.
- Birth-cohort lamb weight weaned per 100 lb of ewe liveweight
- LW_B ÷ (L_E × EW) × 100
- This uses ewes that lambed. Keep the lambing-rate result beside it so open ewes stay visible.
- Not-live-born share of recorded births
- (B_E − A_E) ÷ B_E × 100
- Do not call this stillbirth when total born includes other recorded outcomes.
- Live-born birth-cohort loss to the defined weaning point
- (A_E − W_B) ÷ A_E × 100
- Use only after foster and transfer reconciliation for the same birth cohort.
W_R, the optional rearing-group count, is never substituted into W_B formulas. The weight result uses ewes that lambed, so keep the separate lambing-rate result beside it when comparing whole-flock output.
Set the cohort before looking at the outcome
- 01
Separate ewe age classes
Calculate ewe lambs or first-lambing yearlings separately from mature ewes. Do not hide the age effect inside one flock average.
- 02
Freeze the ewe denominator
Decide which females were eligible and exposed in the breeding window. Keep the same list through scan, lambing, and weaning comparisons.
- 03
Keep birth and rearing links
A fostered lamb can belong to one birth cohort and another rearing group. Record both instead of silently moving it between rates.
- 04
Name each weight point
“Weaned” needs one date or management point. Ewe weight also needs a stated date or stage before comparing seasons or age groups.
Common questions
What does lambing percentage mean?
The phrase is used for more than one calculation. This page names the numerator and denominator instead: ewes lambing per 100 exposed, lambs born per 100 ewes lambing, and lambs born or weaned per 100 ewes exposed.
Should ewe lambs and mature ewes be combined?
Calculate them separately. Ewe lambs and first-lambing yearlings are still growing, often have lower lambing percentages, and their lambs are often lighter at birth. Breed, liveweight, nutrition, and management affect the gap, so the calculator does not apply a universal adjustment.
Which ewes count as exposed?
Use one eligible ewe cohort defined before outcomes are known. Record which females were deliberately exposed in the same breeding window, then keep that denominator unchanged when you compare lambing, birth, and weaning counts.
How should fostered or transferred lambs be counted?
Keep a birth-cohort weaned count separate from the count physically weaned by a rearing group. The calculator uses the birth-cohort count for its weaning rates and shows the rearing-group count only as a reconciliation check.
Keep the dates and counts together
For broader measures, read know your own flock numbers. For the recordkeeping baseline and pre-registered field units, see why keep farm records and the 200-ewe field protocol.
Bring one season’s records to a walkthrough
A founder-run walkthrough can use your current record shape to show setup, a plain-language note, review before save, and export. The 30-day trial requires a card and charges nothing until day 31.