4-20 mA Signal Scaling Calculator
Convert a 4-20 mA loop current to the process value. With the formula, a worked example and typical values.
Try your own numbers
Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
Open in the calculator with these values
About this calculator
This page calculates process value and span from loop current, range at 4 mA and range at 20 mA. Change any value or its unit and the result updates. The steps below show the working for the default values, so you can check each line against your own method.
Limits of this calculator
A current below about 3.8 mA or above 20.5 mA usually signals a wire break or a transmitter fault (NAMUR NE43).
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60381-1, 1982 Ed 2.0 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Industrial analogue DC signal ranges are covered by this standard. The calculator performs a user-defined linear scaling and does not check transmitter, receiver, loop-power or accuracy requirements. |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Loop current | 12 mA |
| Range at 4 mA | 0 |
| Range at 20 mA | 100 |
- Span = (12 − 4) / 16 = 50 %
- PV = 0 + 0.5 × (100 − 0) = 50
Assumptions
- Linear transmitter
- Live-zero 4 mA to 20 mA range
A current below about 3.8 mA or above 20.5 mA usually signals a wire break or a transmitter fault (NAMUR NE43).
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Loop current | mA | 0.004 mA or more, up to 0.02 mA | |
| Range at 4 mA | (number) | any number | |
| Range at 20 mA | (number) | any number |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Process value | (number) | Shown to 3 decimal places | |
| Span (%) | (number) | Shown to 2 decimal places |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
How the result changes with loop current
| Loop current (mA) | Process value | Span |
|---|---|---|
| 4 | 0 | 0 |
| 6 | 12.5 | 12.5 |
| 8 | 25 | 25 |
| 12 | 50 | 50 |
| 16 | 75 | 75 |
| 18 | 87.5 | 87.5 |
| 20 | 100 | 100 |
Other inputs are the same as in the worked example above.
Standards and further reading
These are textbook circuit relationships. The component data sheet sets the real limits, tolerances and ratings, and takes priority over the calculated value.
Frequently asked questions
What do I need to enter?
Enter loop current, range at 4 mA and range at 20 mA. Change the unit of a value with its selector and the result stays the same.
What does this calculator assume?
Linear transmitter. Live-zero 4 mA to 20 mA range.
Which calculators should I use next?
Common next steps are Ohm's Law. The related links at the bottom of this page open them.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- 4-20 mA Loop Formulas: Scaling and Resistor: 4-20 mA current loop formulas: current from process value, process value from current
- Current Shunt Resistor Formulas: Current shunt formulas: the resistance from the full-scale voltage and current, the power at full current
- ADC Resolution and Sampling Formulas: ADC formulas: the size of one step from the bits and the reference, the count for a voltage
More Electronics and Instrumentation calculatorsAll engineering calculators4-20 mA Signal Scaling in Elecmetric