Turndown ratio (also written turn down ratio) is the maximum flow divided by the minimum flow an instrument can measure within its stated accuracy. A 10 : 1 turndown means it works down to 10% of its maximum. Enter your flows below to get the ratio, or enter the maximum and the turndown to find the lowest flow.
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Show reference formula
Q min = Q max ÷ turndown ratio
Q min as % of Q max = 100 ÷ turndown ratio
Use the same flow unit for every value.
| Turndown | Lowest flow, % of max | If max = 100 | If max = 500 |
|---|---|---|---|
| 2 : 1 | 50% | 50 | 250 |
| 3 : 1 | 33.33% | 33.33 | 166.67 |
| 5 : 1 | 20% | 20 | 100 |
| 10 : 1 | 10% | 10 | 50 |
| 20 : 1 | 5% | 5 | 25 |
| 50 : 1 | 2% | 2 | 10 |
| 100 : 1 | 1% | 1 | 5 |
The formula
How to calculate turndown ratio
Turndown ratio compares the top and the bottom of the flow range an instrument can handle. Use the same flow unit for both values.
Turndown ratio: Q max ÷ Q min, written X : 1
Minimum flow: Q min = Q max ÷ turndown ratio
Minimum as % of maximum: 100 ÷ turndown ratio
A 10 : 1 turndown reaches down to 10% of maximum flow, and a 100 : 1 turndown reaches down to 1%.
Worked examples
Turndown ratio examples
Find the ratio
A flow meter measures from 8 m³/h up to 100 m³/h.
100 ÷ 8 = 12.5 : 1, so the lowest flow is 8% of the maximum.
Find the minimum flow
A 200 m³/h meter has a 10 : 1 turndown.
200 ÷ 10 = 20 m³/h, which is 10% of the maximum.
Check your operating point
A 100 m³/h meter at 10 : 1 works from 10 to 100 m³/h. Normal flow is 45 m³/h.
45 m³/h is inside the range, at 4.5 times the minimum. A normal flow of 6 m³/h would fall below it.
Accuracy
Why turndown is not the whole story
Accuracy as % of full scale
±1% of a 100 m³/h full scale is ±1 m³/h at every flow. At 10 m³/h that is ±10% of the reading.
Accuracy as % of reading
±1% of reading at 10 m³/h is ±0.1 m³/h, so the accuracy holds across the whole range.
Check which one the datasheet quotes, and the lowest flow at which the stated accuracy is still guaranteed. That figure, not the headline ratio, is your real turndown.
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FAQ
Turndown ratio questions
What is a good turndown ratio?
It depends on your process. The meter must cover your minimum and maximum flow with margin, and your normal operating flow should sit comfortably inside the range rather than at either edge. A higher ratio matters most where flow varies a lot, for example in batch processes, day and night demand, or start-up and shutdown.
Is it turndown ratio or turn down ratio?
Both spellings mean the same thing, and you will also see turn-down ratio. They all describe maximum flow divided by minimum flow, written as X : 1.
What is the difference between turndown ratio and rangeability?
In flow measurement the two terms are used interchangeably. Rangeability is also the usual word for control valves, where it means the ratio of the largest flow to the smallest flow the valve can control.
How is turndown ratio calculated for a differential pressure flow meter?
Flow varies with the square root of differential pressure, so a 100 : 1 range on the DP transmitter gives only 10 : 1 in flow. That is why orifice plate and other DP systems have a low turndown compared with linear-output meters such as electromagnetic or vortex meters.
How can I increase turndown?
Choose a technology with a wider measuring range, use a smaller meter or line size so velocity stays high at low flow, or fit two meters in parallel, one for low flow and one for high flow. Check the pressure drop at maximum flow whenever you reduce the size.
What turndown does an electromagnetic flow meter have?
It depends on the model and manufacturer, and datasheets often state it as a velocity range rather than a ratio. Compare that velocity range with your process using our mag meter velocity and flow calculator, which converts velocity to flow for your pipe size.
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