Specific Growth Rate (SGR)

In commercial aquaculture, visually estimating the growth of thousands of fish swimming in a murky pond is impossible. If the fish are not growing fast enough to cover the daily cost of their feed, the farm will go bankrupt. Farmers must physically weigh a small sample of the fish regularly. The Specific Growth Rate (SGR) Calculator takes those sample weights and translates them into a daily percentage growth metric. This allows farm managers to objectively evaluate the performance of different feed brands, detect hidden diseases, and mathematically predict the exact date the crop will reach market size.

Understanding the SGR Formula

SGR measures the daily percentage increase in body weight. Because animal growth is exponential (a large fish puts on more absolute grams per day than a tiny fry), you cannot use simple division.

The mathematical formula utilizes natural logarithms: ((ln(Final Weight) - ln(Initial Weight)) / Days) × 100. This complex logarithmic math smooths out the exponential curve, providing a clean, standardized daily percentage (e.g., 2.5% per day) that allows a farmer to compare the growth of a 10-gram fingerling directly against a 500-gram adult.

The Biology of Diminishing Growth

It is biologically impossible to maintain a constant SGR throughout the entire life of the fish. As an animal gets larger, its SGR naturally and inevitably decreases.

A tiny tilapia fry might have an explosive SGR of 8.0% per day. By the time that same tilapia reaches 500 grams, its SGR will drop to 1.0% or 0.5% per day, because the massive fish requires massive amounts of feed simply to maintain its baseline metabolism. Farmers use SGR not as a static goal, but to track whether the fish are following the standard, expected biological growth curve for that specific species.

How to Use the Calculator

You must conduct a physical sampling. Weigh a sample of 50 to 100 fish, divide the total weight by the number of fish, and input that as the Initial Average Weight. Record the date.

Exactly 14 or 30 days later, conduct another sampling and input the Final Average Weight. Input the exact number of days that passed between the two samples. The tool will instantly calculate the SGR. To determine exactly how efficiently the fish converted expensive feed into that new body weight, immediately use our Feed Conversion Ratio (FCR) Calculator.

The Impact of Water Temperature

Fish are cold-blooded (ectothermic). Their metabolism, and therefore their SGR, is dictated entirely by the water temperature.

If a farmer records an SGR of 2.0% in July, and the SGR suddenly drops to 0.5% in November, it does not mean the feed is bad or the fish are sick; the cold water simply shut down their digestive enzymes. Advanced farmers use SGR in conjunction with temperature logs to create a "Thermal-Unit Growth Coefficient," ensuring they do not waste expensive feed during cold weather when the fish physically cannot grow.