Arduino map() and constrain() — turn any reading into the range you need

#The short answer

map(value, fromLow, fromHigh, toLow, toHigh) takes a number from one range and gives back the matching number in another range.

int brightness = map(reading, 0, 1023, 0, 255);

If reading is halfway (about 512), brightness is halfway too (127). constrain(x, low, high) then keeps a number inside limits, so a reading a little outside the range can never give a silly result.

#Why you need it

Every input and output on the UNO speaks its own "range":

To make a knob control a servo, you need 1023 to become 180, 512 to become 90, and 0 to stay 0. That is exactly what map() does.

#The formula inside

map() is one line of school maths:

result = (value − fromLow) × (toHigh − toLow) ÷ (fromHigh − fromLow) + toLow

Let's try map(512, 0, 1023, 0, 180):

Half of the knob is half of the servo's swing. ✅

#Example 1: a knob dims an LED

This is the code from our Knob project:

int reading = analogRead(KNOB);                 // 0 … 1023
int brightness = map(reading, 0, 1023, 0, 255); // 0 … 255
analogWrite(LED, brightness);

analogWrite() only understands 0 to 255, so without map() a reading of 1023 would be far too big. (A quick shortcut you will also see is reading / 4, which is nearly the same.)

Play in 3DRun the code · press the parts · turn it around

#Example 2: ranges can run backwards

map() doesn't care which way a range goes. Our Night light wants the LED brighter when it is darker:

analogWrite(LED, map(light, 0, DARK, 255, 30));

When light is 0 (pitch black) the result is 255, full brightness. When light reaches DARK (400), the result is 30, a soft glow. The "to" range simply goes downwards.

The soil sensor does the same with the "from" range: wetter soil reads lower (about 260 in water, 520 in dry air), so the plant monitor uses map(raw, DRY, WET, 0, 100) with DRY = 520 and WET = 260.

#constrain(): keep it inside the lines

map() does not stop at the ends of the range. If the soil is even drier than your "dry" reading, say 560, then:

map(560, 520, 260, 0, 100) = −15 %

A negative percentage on the screen looks broken. constrain() fixes that:

int percent = constrain(map(raw, DRY, WET, 0, 100), 0, 100);

This is the exact line from our Plant watering monitor. Use constrain() whenever a sensor's real readings might go past the numbers you measured.

#The catch: whole numbers only

map() works with whole numbers (long integers). It cuts off any fraction; it does not round.

• map(1, 0, 10, 0, 3) gives 0, not 0.3.
• map(5, 0, 10, 0, 3) gives 1, not 1.5.

For LED brightness or servo angles that is fine. If you need decimals, for example volts from a reading, do the maths with float yourself:

float volts = analogRead(A0) * 5.0 / 1023.0;   // 0.00 … 5.00 V

#Common mistakes

• The output goes the wrong way. The "from" or "to" range is upside down. Swap the two numbers of one range.
• Values past the ends. map() does not limit its result. Wrap it in constrain().
• Decimals disappear. map() is integer-only. Use float maths for volts and temperatures.
• The servo jitters at the ends. Your knob may never reach exactly 0 or 1023. Measure its real ends in the Serial Monitor and use those numbers.

#Check yourself

#Questions people ask

#Does Arduino map() work with decimals?

No. map() uses whole numbers and drops anything after the decimal point. For decimals, write the formula yourself with float variables.

#Can map() reverse a range?

Yes. Put the numbers in the opposite order, for example map(x, 0, 1023, 255, 0), and big inputs give small outputs.

#Why is my map() result bigger than the range?

map() does not limit its output. If the input goes beyond fromLow–fromHigh, the output goes beyond toLow–toHigh. Use constrain() around it.

#What is the difference between map() and constrain()?

map() converts a number from one scale to another. constrain() doesn't convert anything; it only clips a number to a minimum and maximum. They are often used together.

#What's next

Build the Knob project to see map() live, then try the Plant watering monitor, which uses map() and constrain() together.

Last updated 2026-11-07