Transmitter Scaling Converter (Input <> Output mA)

Every 4–20 mA transmitter loop has a precise mathematical relationship between the process value and the current on the wire. This free tool calculates that relationship instantly — in either direction, to five significant figures.

Enter a process value (engineering units) and get the correct mA output. Enter a mA reading and get the corresponding process value. Enter both and see the error between them as % of span, with a PASS or FAIL result against your tolerance setting.

Set your LRV, URV, and units at the top — the tool updates automatically. Use the Quick Reference table to screenshot target values before heading to the field. Works on your phone at the transmitter. No login, no app download.

EU ↔ mA Converter — Orion Technical Solutions

EU ↔ mA Converter

4–20 mA loop  ·  Engineering unit conversion  ·  % span error

Orion Technical Solutions LLC  ·  orion-technical.com

📱 Rotate phone sideways for best view

What this tool does

This converter instantly calculates any value on a standard 4–20 mA instrumentation loop given any other value. You can work in either direction — enter an engineering unit (EU) value and get the mA output, or enter a mA reading and get the EU value. You can also enter two values and see the error between them.

TRANSMITTER RANGE LRV = 0°F URV = 200°F Units Tolerance ±% ← Set first EU → mA 100 °F Enter EU value here 50.000% 12.00000 mA Results appear automatically ↑ mA → EU Enter mA — — COMPARE (optional) Enter your actual measured value → see error + PASS/FAIL result QUICK REFERENCE TABLE — common % points auto-calculated
Tool layout overview

The four things you can do with this tool:

  1. Convert EU to mA — enter any engineering unit value, get the correct mA output and % of span
  2. Convert mA to EU — enter any mA reading, get the corresponding EU value and % of span
  3. Check error — enter a target value in one column and your actual measured value in the compare field — get error in mA, EU, and % of span, with PASS or FAIL against your tolerance
  4. Quick reference — the table at the bottom auto-calculates all standard % points for your range so you can look up target values fast
Start here: Set your LRV, URV, units, and tolerance at the top first. Everything else updates automatically.

Converting EU to mA

Use this when you know the process value (in engineering units) and need to know what the transmitter mA output should be at that point.

EU → mA 100 °F % OF SPAN 50.0000% mA OUTPUT 12.00000 ↑ Results appear instantly ← Type here ← Read result
Enter EU in the large box — results appear automatically
Step by step:
1. Make sure your LRV, URV, and units are set at the top
2. Tap the large box in the EU → mA column
3. Type the engineering unit value (e.g. 100 for 100°F)
4. Read the % of span and mA output results below — they update as you type

Example — 0 to 200°F transmitter:

  • Enter 0°F → 0.000% → 4.00000 mA
  • Enter 100°F → 50.000% → 12.00000 mA
  • Enter 200°F → 100.000% → 20.00000 mA
  • Enter 75°F → 37.500% → 10.00000 mA
Tip: You can enter values outside the 0–100% range. The tool calculates correctly for any EU value — useful for checking transmitter behavior outside normal range or verifying cutoff behavior.

Converting mA to EU

Use this when you have a mA reading on your loop calibrator or meter and need to know what process value it corresponds to.

mA → EU 12.00 mA % OF SPAN 50.0000% °F VALUE 100.0000 ↑ Results appear instantly Type here →
Enter mA in the large box — EU value appears automatically
Step by step:
1. Tap the large box in the mA → EU column
2. Type the mA reading from your meter or calibrator
3. Read the % of span and EU value results below

Example — 0 to 200°F transmitter:

  • Enter 4.000 mA → 0.000% → 0.0000°F
  • Enter 12.000 mA → 50.000% → 100.0000°F
  • Enter 20.000 mA → 100.000% → 200.0000°F
  • Enter 12.048 mA → 50.300% → 100.6000°F
Common use: You're at a transmitter with a loop calibrator. It reads 14.72 mA. Enter that and instantly know the process value — no mental math needed.

Checking calibration error

This is the most powerful feature. Enter a target value in the main box and your actual measured value in the Compare field. The tool calculates the error three ways and tells you PASS or FAIL against your tolerance setting.

EU → mA 100 °F 50.0000% 12.00000 mA COMPARE — enter actual measured mA 12.048 FAIL +0.048 mA | +0.600°F | +0.300% span Limit: ±0.250% span (±0.040 mA) ← Target EU ← Ideal mA ← Actual mA ← Error details
Target EU in main box, actual measured mA in compare field → instant PASS/FAIL
EU → mA error check (most common at field):
1. Enter your target EU in the EU→mA box (e.g. 100°F at 50%)
2. Read the ideal mA shown (e.g. 12.00000 mA)
3. Apply that EU input to the transmitter, measure the actual mA output
4. Enter your actual measured mA in the Compare field
5. See the error in mA, EU, and % of span — with PASS or FAIL
mA → EU error check (when starting from a mA reading):
1. Enter your mA reading in the mA→EU box
2. Read the ideal EU shown
3. Enter your actual EU reading (from HART PV or HMI) in the Compare field
4. See the error and PASS/FAIL
Smart transmitter tip: Always compare the HART digital PV to the applied EU input first. If the digital PV is wrong, the mA output will also be wrong — and the error will be in the A/D section, not the output section. Do not adjust output trim to fix an A/D error.

Quick Reference Table

The table at the bottom of the tool automatically calculates target EU values, ideal mA values, and tolerance bands (±EU and ±mA) for the standard calibration check points: 0%, 10%, 25%, 50%, 75%, 90%, and 100% of span.

% Span EU (°F) mA ±EU Tol ±mA Tol 0% 0.0000 4.00000 ±0.5000 ±0.04000 25% 50.0000 8.00000 ±0.5000 ±0.04000 50% 100.0000 12.00000 ±0.5000 ±0.04000 Blue row = nearest point to your current EU entry in the converter above
Quick reference table — updates automatically when you change the range

The table updates instantly whenever you change the range at the top. The row nearest to whatever EU value you currently have entered in the converter is highlighted in blue — making it easy to cross-reference.

Field use: Before going to the transmitter, set your range and tolerance, then screenshot the quick reference table. You'll have all the target mA values and tolerance limits in your pocket without needing to be connected to the internet.

Tolerance column explained: The ±EU Tol and ±mA Tol columns show the maximum allowable deviation at any point. These are the same value at every row — tolerance is always a fixed window based on the total span, not the reading at that point.

Transmitter Range
Span: 200.0 °F  |  Sensitivity: 0.08000 mA/°F  |  Tol: ±0.5000 °F / ±0.04000 mA
EU → mA
°F
% of Span
—
mA Output
—
Compare — enter actual mA measured:
Actual mA —
mA → EU
mA
% of Span
—
°F Value
—
Compare — enter actual EU reading:
Actual °F —
Quick Reference — % Points
% Span EU (°F) mA ±EU Tol ±mA Tol

What is % of span?

% of span is where a value falls within the transmitter's configured range, expressed as a percentage from LRV (0%) to URV (100%). All calibration tolerances are stated as % of span — not % of reading.

Formula: % of span = (EU value − LRV) ÷ (URV − LRV) × 100
Example: 0–200°F range, 75°F → (75−0)÷200×100 = 37.500% of span

EU to mA conversion

The 4–20 mA loop maps your EU range linearly to the current range. 4 mA = LRV (0%), 20 mA = URV (100%).

EU → mA: mA = ((EU − LRV) ÷ (URV − LRV)) × 16 + 4
mA → EU: EU = ((mA − 4) ÷ 16) × (URV − LRV) + LRV
Sensitivity: 16 ÷ (URV − LRV) mA per EU unit

What does the tolerance band mean?

±0.25% of span means the allowable error is ±0.25% of the full configured range — the same window applies at every point from 0% to 100%.

For 0–200°F at ±0.25% tolerance:
±0.25% × 200°F = ±0.500°F  |  ±0.25% × 16 mA = ±0.040 mA
The ±0.500°F window applies at 0°F, 100°F, and 200°F equally.
Common mistake: Applying tolerance as % of the reading rather than % of span. These are not the same thing at low inputs. Always use % of span unless the spec explicitly says otherwise.

Using the comparison fields

Enter a target EU on the left → get ideal mA → enter your actual measured mA in the compare field → see the error in mA, EU, and % span with a PASS/FAIL result.

The same works right-to-left: enter a measured mA → get ideal EU → compare to actual EU reading.

Smart transmitter note

On a smart (HART) transmitter, a correct mA output at the zero point does not prove the transmitter is calibrated correctly. If the LRV register has been offset to mask an A/D error, mA will appear correct at 0% only — while the digital PV and mA are wrong everywhere else in the range. Always check the digital PV via HART communicator or LCD when verifying smart transmitters.

Common 4–20 mA error types

  • Zero offset (constant error at all points): A/D drift on smart Tx — perform Sensor Trim. Zero pot on analog Tx.
  • Span error (error grows with input): Check digital PV first. If PV correct but mA wrong — Output Trim. If PV also wrong — Sensor Trim.
  • mA low at AI card vs. terminals: Field wiring leakage to ground. Not a transmitter issue.
  • mA high at AI card vs. terminals: Moisture between + and − conductors in J-box. Not a transmitter issue.
  • EU wrong at HMI, mA correct: AI card calibration drift or DCS/PLC scaling error. Do not adjust transmitter.
Orion Technical Solutions LLC  ·  orion-technical.com
Free I&C field tools  ·  No login required

Mike Glass

About the author

Mike Glass

Mike Glass is an ISA Certified Automation Professional (CAP) and a Master Certified Control System Technician (CCST III). Mike has 40 years of experience in the I&C industry performing a mix of startups, field service and troubleshooting, controls integration and programming, tuning & optimization services, and general I&C consulting, as well as providing technical training and a variety of skills-related solutions to customers across North America.

Mike can be reached directly via [email protected] or by phone at (208) 715-1590.