Flock uniformity (CV%) — how many birds to weigh and how to calculate step by step
A uniform flock means better results and better payment at slaughter. We explain in plain language what flock uniformity is, how to measure it with CV% and percentage uniformity, how many birds to weigh, and what to do with the result.
verifiedFrom the team that has organised work on poultry farms for years.
Are the birds in your house growing at a similar rate? Is one bird twice the weight of another? The answer comes from two numbers: CV% (coefficient of variation — how much, on average, individual bird weights differ from the flock average, relative to that average) and percentage uniformity (what proportion of birds fall within ±10% of the average weight). More about broiler rearing in general is in the guide on broiler farming.
Why flock uniformity matters
A uniform flock is first and foremost about money: the slaughterhouse pays best for birds in the right weight range. Birds that are too light or too heavy fall outside that range — you get less, or nothing. Beyond that, uniform birds are easier to feed (one feeding programme fits the whole flock), and treatment and transport logistics are simpler. Non-uniformity shows up early: if body weight on day 7 is already spread wide, the problem grows throughout the whole rearing period.
Two uniformity measures — simply explained
CV% (coefficient of variation) = (standard deviation of weight ÷ average weight) × 100. Standard deviation is a number that tells you how much, on average, individual bird weights differ from the flock average — the smaller it is, the closer the weights are to one another. Divide it by the average and multiply by 100 to get a percentage. The lower the CV%, the more uniform the flock. Percentage uniformity = the share of birds whose weight falls within ±10% of the average. The higher, the better. Both measures describe the same thing from different angles: CV% looks at the spread of weights, uniformity looks at how many birds are "in range".
Where do the numbers in this guide come from?
The indicative values (CV% thresholds, uniformity thresholds) are based on Aviagen guides for the Ross 308 line and standard zootechnical practice. They may vary between lines and production systems — treat them as a reference, not a fixed norm. You can track body weight and uniformity week by week using the digital broiler flock card.
Calculating CV% and flock uniformity — from weighing to assessment
- 1
Select birds for the sample — around 50–100 birds
The minimum sample is roughly 50 birds, and ideally around 100 or more. The key point is to not only weigh birds near the house door — that is not a representative sample. Walk the whole house and catch birds from different areas: corners, the centre, under the drinker lines and under the feeder lines. A random selection from the whole house gives a reliable result. Weigh birds at the same time each week (ideally in the morning, before or after first feeding — but always consistently) so results are comparable between weeks.
- 2
Weigh each bird individually and record the result
Each bird individually — do not weigh several together and divide. Record weights in grams (e.g. 2,150 g, 1,980 g, 2,340 g). You can record on paper, in an Excel table or directly in the DlaFerm.pl app, which will do all the calculations for you. Check that the scales are zeroed before each weighing — a tarring error distorts all results. If you use scale integration, weights upload automatically.
- 3
Calculate the average weight
Add up all the recorded weights and divide by the number of birds. Example: you weighed 5 birds: 1,900 g, 2,100 g, 2,000 g, 2,200 g, 1,800 g. Sum = 10,000 g. Average = 10,000 ÷ 5 = 2,000 g. With a real sample of 50–100 birds, a calculator or the app will do this for you.
- 4
Calculate the standard deviation
Standard deviation tells you how much, on average, bird weights differ from the average. Step by step: (1) subtract the average from each weight and square the result — e.g. (1,900 − 2,000)² = 10,000; (2,100 − 2,000)² = 10,000; (2,000 − 2,000)² = 0; (2,200 − 2,000)² = 40,000; (1,800 − 2,000)² = 40,000; (2) add these squares: 10,000 + 10,000 + 0 + 40,000 + 40,000 = 100,000; (3) divide by the number of birds minus 1 (i.e. 4): 100,000 ÷ 4 = 25,000; (4) take the square root: √25,000 ≈ 158 g. That is the standard deviation. In practice you calculate this in Excel (STDEV function) or in the app — this shows you where the number comes from.
- 5
Calculate CV%
CV% = (standard deviation ÷ average weight) × 100. From the example: (158 ÷ 2,000) × 100 = 7.9%. That is very good flock uniformity. The smaller the CV%, the more similar the birds are in weight. As a guide: below about 8% = very uniform; 8–10% = good; above about 12% = non-uniform, worth investigating the cause.
- 6
Calculate percentage uniformity
Count how many birds in the sample weigh between average × 0.90 and average × 1.10 (i.e. ±10% of average). From the example: 90% of 2,000 g = 1,800 g; 110% of 2,000 g = 2,200 g. Check each weight: 1,900 g — yes (within range); 2,100 g — yes; 2,000 g — yes; 2,200 g — yes (exactly on the boundary, counted); 1,800 g — yes (exactly on the boundary, counted). All 5 birds are within range → uniformity = 5 ÷ 5 × 100 = 100%. As a guide: above about 80% = good; 70–80% = caution; below about 70% = problem.
- 7
Assess the result and decide what to do
Compare CV% and percentage uniformity against the assessment table below. If results are worse than expected, look for the cause: uneven feeding (blocked feeder positions, uneven drinker line pressure), a health problem affecting part of the flock, uneven house heating, poor bird distribution. In subsequent weighings, track the trend — is uniformity improving, declining or holding steady? You can record and track weekly results in the digital broiler flock card.
How to assess flock uniformity — indicative thresholds
Indicative values based on Aviagen Ross 308 guides and zootechnical practice. Treat them as a reference — they may vary between lines and production systems.
CV% below about 8% — very good
A very uniform flock. Birds are growing similarly, feeding is efficient and you can expect good settlement at slaughter. Maintain the conditions that produced this result.
CV% 8–10% — good
Flock uniformity is within an acceptable range. Monitor in subsequent weeks — if CV% is rising, look for the cause sooner rather than waiting for slaughter.
CV% 10–12% — caution
Uniformity is starting to become problematic. A larger proportion of birds will fall outside the target slaughter weight range. Check feeding, watering and flock health.
CV% above about 12% — non-uniform flock
High weight variation. At slaughter a significant proportion of birds may fall outside the target range, leading to lower payment. Investigate the cause and consult an adviser.
Common mistakes when weighing and calculating flock uniformity
Incorrect weighing gives a false picture of the flock. Below are the mistakes most easily made and most likely to distort the result.
Weighing only near the house door
Birds near the door are often the more timid, lighter ones — chosen by those less willing to compete at feeders. A sample only from the entrance underestimates the average. Walk the whole house and catch birds from different locations.
Sample too small
A sample of 10–20 birds gives a very unreliable result — a few extreme values can shift CV% significantly. As a guide, the minimum is about 50 birds, ideally around 100 or more. The larger the sample, the more reliable the CV% and uniformity result.
Scales not zeroed or not calibrated
A tarring error shifts all results — for example a forgotten container on the pan or scales placed on an uneven floor. Zero the scales before each weighing series and check them with a test weight if you have one.
Weighing at different times of day
A bird's weight changes through the day depending on when it last ate and drank. Weights taken in the morning before feeding and in the afternoon after feeding will give different results. Always weigh at the same time, ideally in the morning.
Frequently asked questions about flock uniformity and CV%
What is CV% and why is a lower value better?add
CV% (coefficient of variation) = (standard deviation of weight ÷ average weight) × 100. It tells you by what percentage, on average, individual bird weights differ from the flock average. The lower the CV%, the more similar the birds are in weight — the flock is uniform. A high CV% means weights are very spread out: some birds weigh far more than others. This leads to worse payment at slaughter and more difficult feeding.
What is the difference between CV% and percentage uniformity?add
CV% looks at the spread of all weights — it takes into account every deviation from the average, regardless of direction. Percentage uniformity only looks at how many birds fall within ±10% of the average — it ignores how far outside that range the outliers are. Both measures describe the same phenomenon but in slightly different ways. In practice both are used together.
How many birds need to be weighed for a reliable result?add
As a guide, the minimum is about 50 birds, ideally around 100 or more — taken randomly from different locations in the house, not just near the door. The larger the sample, the smaller the effect of random extreme values on the CV% and uniformity result. For very large flocks (tens of thousands of birds), a sample of 100–150 birds from several points in the house is sufficient.
When during the rearing cycle should flock uniformity be measured?add
Weighing is usually done once a week — often on the same day of the week and at the same time of day. The first check weighing is often around day 7 (body weight on day 7 is an important indicator of how the flock has started). Then weekly weighings through to slaughter. Comparing results across successive weeks lets you track the uniformity trend.
What are the indicative CV% thresholds for good flock uniformity?add
As a guide: below about 8% = very good uniformity; 8–10% = good; 10–12% = starting to be problematic; above about 12% = non-uniform flock, worth investigating. For percentage uniformity (±10% of average): above about 80% = good; 70–80% = caution; below about 70% = problem. These values are indicative and may differ between genetic lines.
What should I do when CV% is high and the flock is non-uniform?add
Look for the cause: (1) feeding — check that all feeders are working and have equal access, that drinker line pressure is even throughout the house; (2) health — is part of the flock sick or lame, limiting access to feed; (3) environment — is the temperature uniform, are there cold or hot zones forcing birds to cluster; (4) stocking density — too high a density increases non-uniformity. Consult a zootechnical adviser or veterinarian.
Can the DlaFerm.pl app automatically calculate CV% and uniformity?add
Yes — after entering weights (manually or through scale integration) the app automatically calculates the average, standard deviation, CV% and percentage uniformity. Results are saved in the digital flock card, so you can see the trend across successive weeks without any manual calculations.
Calculate flock uniformity in DlaFerm.pl — no Excel needed
Enter your bird weights and the app calculates CV%, percentage uniformity and saves the result to your digital flock card. Join the farmers who no longer calculate by hand. Create a free farm account or write to us.
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