How this instrument works
A milliradian, mil for short, is an angle: one thousandth of a radian, small enough that a target one metre tall spanning exactly one mil in the reticle sits precisely one thousand metres away. That single fact is the whole instrument. For a narrow angle the arc a target subtends is almost indistinguishable from the straight height crossing it, so height divided by range very nearly equals the angle in radians; multiply both sides by the 1000 mils in a radian and rearrange, and range = height × 1000 ⁄ mils falls straight out of that small-angle approximation.
Rangefinding reticles turn this into a field technique used well beyond military scopes: a hunter judging a deer at last light, a birder sizing up a raptor on a fence post, a forward observer walking artillery onto a grid square, a sailor judging distance to another vessel by its known mast height. The one thing every user must supply honestly is a real height for the target — a fence post, a vehicle roofline, an average adult's shoulder height — because the formula trusts that entry completely and has no way to check it against the scene.
The small-angle approximation this depends on degrades as the mil reading grows, since the true relation involves a tangent rather than a straight ratio; by roughly 100 mils, about 5.7 degrees, the error passes half a percent and keeps climbing, so the method suits distant, narrow subtensions rather than close objects filling much of the reticle. It also assumes a true milliradian — some military optics instead use a NATO mil defined as 1/6400 of a circle, about 1.8 percent smaller, and confusing the two shifts every range estimate by that same fraction.
- Enter the target's true height — a person, vehicle, or landmark of known size — into Target height, choosing meters, feet, or yards.
- Count how many mils that same target spans across your reticle's mil-dot or MRAD scale, and type that number into Reticle reading, mils.
- The formula multiplies Target height by 1,000 and divides by the mils you entered; read the answer straight off Estimated range.
- Switch the Estimated range unit menu to yards or kilometers to match your rangefinder, map, or artillery grid square.
Worked example — ranging a 1.8 m tall target at 10 mils
A person of average height, 1.8 m, stands in a field and spans exactly 10 mils across a binocular reticle's mil-dot scale. The instrument computes range = 1.8 × 1000 ⁄ 10 = 180 m — a genuinely useful figure before deciding whether a shot, a stalk, or a radio call comes next.
Halve the mil reading and the range estimate doubles, because the two are inversely proportional: the same 1.8 m target spanning only 1 mil, a much smaller image in the glass, sits ten times farther off, at 1,800 m. That inverse relationship is the entire reason a mil-dot reticle is useful for judging distance without a laser — read the subtension, do the division, and the range falls out.
Questions
What exactly is a mil in a rangefinding reticle?
A mil is short for milliradian, one thousandth of a radian, and by definition a target spanning exactly one mil sits at a range one thousand times its own height or width. A mil-dot or MRAD reticle simply gives you a ruler marked in that unit, so the instrument only has to turn a subtension you can count into a distance you can use.
Why does the formula multiply by 1000 instead of some other number?
Because a milliradian is defined as 1/1000 of a radian, and for small angles a target's height very nearly equals the range times the angle in radians. Converting that angle to mils multiplies it by 1000, so isolating range for a known height and mil reading brings the same 1000 back out: range = height × 1000 ⁄ mils.
How accurate is mil-relation ranging in the field?
Accurate to within a percent or two at typical hunting and shooting distances, provided the entered height is correct, since the arithmetic itself is exact. Most real-world error comes from misjudging the target's true height rather than from the formula — a 10 percent height error becomes a 10 percent range error directly.
Does it matter if I enter height in feet instead of meters?
No — Target height accepts meters, feet, or yards, and Estimated range can be read out in meters, yards, or kilometers independently of whichever unit height was entered in. The instrument converts internally before dividing, so the mil reading is the only figure that must be read directly off the reticle.
What is the difference between a true mil and a NATO mil?
A true milliradian is exactly 1/1000 of a radian, giving 6,283.2 mils per full circle; a NATO mil, used in some artillery and older military optics, is instead defined as 1/6400 of a circle, about 1.8 percent smaller. Mixing the two into one range estimate shifts the answer by that same 1.8 percent, which matters at long range but rarely at hunting distances.
Why does entering a zero height give a zero range?
Because the formula is a direct proportion, range = height × 1000 ⁄ mils, and multiplying by a height of zero always returns zero, regardless of the mil reading. It is a mathematically correct but practically meaningless result, a reminder that the instrument only ever repeats back what the target height field tells it.