LM317 Calculator

How to use this LM317 calculator

What this calculator does

This calculator solves the standard resistor-divider relationship used to program an LM317 adjustable positive voltage regulator. It can calculate Output voltage from Resistor (R1) and Resistor (R2), or solve backward for either resistor when you know the other resistor and your target output voltage. The model uses a 1.25 V reference and neglects the small adjust-pin current, matching the common simplified design equation. Texas Instruments shows the complete equation, including the adjust-current term, in the LM317 datasheet and application circuit. This calculator is for set-point arithmetic; it does not determine input headroom, heat-sink size, load current capability, capacitor selection, tolerance stack-up, or thermal safety.

When to use it

Use it when selecting a resistor pair for a bench supply, checking the voltage created by resistors already fitted to a board, estimating a replacement resistor during a repair, or teaching how the LM317 feedback divider sets voltage. It is especially useful for quick what-if checks before you round a theoretical resistance to a standard resistor value.

How to calculate

1. The calculator opens with a ready demonstration: Calculate = Output voltage, R1 = 240 Ω, and R2 = 1200 Ω. The result is 7.50 V, and the Excel workbook is immediately available.
2. Use Calculate to choose Output voltage, Resistor R1, or Resistor R2. The quantity being solved becomes read-only; the other two become editable source values.
3. Enter resistances in ohms and voltage in volts. Use ordinary decimal notation with a period, such as 240, 1200, 7.5, or 12.00. Commas, scientific notation, unit letters inside the field, negative resistance, and malformed values are rejected rather than silently reinterpreted.
4. Read the large live result, then check the three summary values and the displayed equation. The range note tells you whether the calculated output is inside the 1.25 V to 37 V adjustable range published for the TI LM317.
5. Select Download Excel to export the current validated inputs and result as a genuine XLSX workbook. Reset clears the demonstration and all input data; after Reset, Excel export stays disabled until you enter a complete valid state again.

Input guide

Calculate is required and selects which quantity is unknown. Choose Output voltage when both resistors are known; choose Resistor R1 when R2 and a target voltage are known; choose Resistor R2 when R1 and a target voltage are known. Changing this selector changes which field is read-only, not the underlying equation.

Resistor (R1) is the resistor connected between the LM317 output and adjust nodes in the conventional divider. Enter a positive finite resistance in ohms; 240 Ω is the startup example. A larger R1, with R2 unchanged, lowers the ratio R2/R1 and therefore lowers the calculated output voltage. A common mistake is swapping R1 and R2 in the formula. Resistor (R2) is the resistor from adjust toward ground in the conventional circuit. Enter a non-negative finite resistance in ohms; 1200 Ω is the example. Increasing R2 with R1 fixed raises the output voltage. R2 = 0 Ω gives the simplified minimum of 1.25 V when Output voltage is being calculated.

Output voltage is entered in volts only when you are solving for R1 or R2. A realistic example is 7.5 V. The target must be at least 1.25 V to produce a non-negative R2 under the simplified equation, and solving R1 requires a target above 1.25 V when R2 is positive. Values above 37 V may still produce a mathematical resistor result, but the range note flags that the target is outside the published adjustable-output range; do not treat that arithmetic result as an operating recommendation.

Output guide

The large Live result is whichever quantity you selected under Calculate. The R1, R2, and Vout summary pills and result cards repeat the complete solved state so you can verify the relationship at a glance. Resistance results are displayed in Ω or kΩ for readability but remain the same resistance value; voltage is displayed in volts. The equation line substitutes the current numbers into the active form of the model. The range status is a device-range check, not a guarantee of regulation: actual performance also depends on input-output differential, current, temperature, tolerances, and implementation.

Worked example

With R1 = 240 Ω and R2 = 1200 Ω, the simplified equation is Vout = 1.25 × (1 + R2/R1). The ratio is 1200/240 = 5, so Vout = 1.25 × (1 + 5) = 1.25 × 6 = 7.50 V. That is the exact first-open result shown above and the same numeric value exported in the startup workbook. If you instead choose Resistor R2, keep R1 at 240 Ω and enter 7.5 V; the rearranged equation returns R2 = 240 × (7.5/1.25 – 1) = 1200 Ω.

For device-level limits and design details beyond the resistor equation, review Texas Instruments' LM317 product specifications, including output range, current capability, line/load regulation, and protection features.