4-20 mA Scaling Calculator

Back to Resources

A standard current loop maps 4 mA to the lower range value (LRV) and 20 mA to the upper range value (URV) linearly. This calculator works both directions: enter a process value to get the current the transmitter should produce, or enter a measured current to get the value it represents. Percent of span is shown for both.

The Formula

The loop is linear between the two endpoints. From process value to current:

Inverted, from current to process value:

Percent of span is the position of the current within the 16 mA live zero band:

Worked example. A pressure transmitter is ranged 0 to 250 psi, so LRV = 0 and URV = 250. At 100 psi the current is 4 + 16 × (100 - 0) / (250 - 0) = 4 + 16 × 0.4 = 10.4 mA. Reading it back, a measured 10.4 mA gives 0 + 250 × (10.4 - 4) / 16 = 250 × 0.4 = 100 psi, at 40% of span.

Assumptions and Limits

• The scaling is strictly linear, with 4 mA at the LRV and 20 mA at the URV. Square-root extraction (used on some flow transmitters) is not applied here; scale in linear units first.
• Values below 4 mA or above 20 mA are computed anyway so you can check over-range and under-range behavior, but a real transmitter may saturate or drive a fault current instead.
• Results are engineering estimates for setup and checkout. Verify against the transmitter's own configuration, the loop drawing, and a measured reading at a known input.

FAQ