Extend your arm straight and raise your thumb. Close your left eye, align your right eye, thumb, and the target in a straight line. Then close your right eye and open your left eye. Remember the point on the right side of the target where your left eye, thumb, and extended line meet. Estimate the distance between that point and the target, then multiply by 10 — this gives you the approximate distance to the target. Raise your thumb. With your arm extended horizontally, the angle between your line of sight through your fingertip and the horizontal line is about 120 mils. Estimate the angle between the lines of sight passing over the top and bottom of the target as a number of mils, then multiply by the target’s height (based on experience) to get the distance.
For example, the average height of a Japanese soldier is about 1.5 meters. The angle between the line of sight passing over their head and foot is about 100 mils, making the distance approximately 150 meters. Usually, if the length of certain objects is known — such as cars or buildings — you can estimate the distance based on the angular size they occupy in your field of view (measured in mils in military surveying).
The number of mils obscured by your extended thumb is fixed. By comparing this with the target being measured, you can determine this angular value. After conversion, you can get an approximate distance. A mil is one-six-thousandth of a full circle. Another method: extend your right arm, close your left eye, and align an object so it just touches the left side of your thumb. Without moving your hand, switch eyes — you’ll notice the object appears to jump a short distance. Multiply the ratio of the object’s estimated width to the jump distance by 50 to get an approximate distance. Experience also plays a role. Mils calculation is essentially about determining the corresponding mil value of a real object in your hand and then calculating the approximate distance.
In theory, you can measure distance by extending your arm, raising your thumb, and using similar triangles based on the distance from your eye to your thumb (approximately the length of your arm), the length of your thumb, and the height of the object being measured. But in practice, using this skill relies heavily on experience, and it takes long-term practice to master. Also, it's important to clarify that finger-based distance measurement is mostly used for marching and rough artillery positioning, and is a required skill for all soldiers. Snipers rarely use finger measurements because extending the arm can easily expose them — snipers usually rely on direct visual estimation. Finger measuring can generally estimate distances of 2–4 kilometers (when there are clear landmarks like houses or trees), with experienced soldiers having an error margin of no more than 200 meters. Visual estimation is typically used for distances within 1 kilometer, with errors within 50 meters. Both finger measuring and visual estimation require long-term practice, familiarity with the sizes of common objects, how they appear visually at different distances, and the ability to skillfully compare with reference objects of known distance. With dedicated practice, you’ll surely succeed.
The “Thumb Distance Measurement Method” is based on right-angle trigonometric functions. To measure the distance to a target N meters away: 1. Hold your right arm straight out to the side, make a fist, and raise your thumb. 2. With your right eye (left eye closed), align the left edge of your thumb with the target in a straight line. 3. Keep your arm and thumb still, close your right eye, and look with your left eye — you’ll see the left edge of your thumb has moved away from the right edge of the target. 4. Estimate this distance (you can also measure it), then multiply it by 10 — the result is your approximate distance to the target.
Also included are commonly used visual distance measurement methods (the “Jump Eye Method”) in the Chinese and U.S. militaries. Legend has it that Chinese military and police snipers commonly use the jump eye method: make a tight fist with your right hand, thumb pointing up, extend your right arm, close your left eye, and use your right eye to align the side of your thumb with the target. Keeping your posture, close your right eye and use your left eye to observe another reference object along the same side of your thumb — the distance between the reference object and the target, multiplied by 10, gives the actual distance to the target. The U.S. military method is somewhat similar: make a tight fist with your right hand, lay your thumb flat, extend your right arm, close your left eye, and use your right eye to align the upper side of your thumb with the target. If the target is just the width of one step, you’re 100 yards away; if it’s two steps, you’re 200 yards away… … Other visual distance estimates based on objects rely on eyesight, target clarity, and practical experience to judge distance. Basic methods include the comparison method and judgment method.
(1) Comparison Method. This involves comparing the distance to be measured with a known distance (like the distance between utility poles, a previously measured distance, or familiar lengths such as 100m, 200m, 500m, etc.) to calculate the distance. You can also halve the distance or break it into several segments, compare them section by section, and calculate the total. (2) Judgment Method. This involves estimating distance based on the clarity of the target. Under normal vision and weather conditions, the distance at which targets can be distinguished can be referred to in the table below. However, since individual eyesight varies, use this table flexibly based on your own experience.
Distance (meters) Target Clarity
100 Facial features, finger joints, external parts of infantry weapons are visible
150-170 Buttons on clothing, canteens, small details of equipment are visible
200 Roof tiles, leaves, wires on barbed wire are visible
250-300 Cracks in walls, individual wires in roof tiles can be counted; facial features are unclear, but clothing colors can be distinguished
400 Faces are unclear, but heads and shoulders can be distinguished
500 Door openings and window grids are visible, but individual tile grooves are not clear; heads and shoulders are indistinct, but gender can be distinguished
700 Roof tiles appear stringy, window frames are visible, pedestrian leg movements can distinguish left from right, but elbows are indistinct
1000 House outlines are clear but roof tiles are messy, doors appear square and window frames vanish; human bodies appear uniformly thick
1500 Roof tiles are flat and glossy, windows appear as holes; people are small black dots, movement is indistinguishable
2000 Windows are dark shadows, doors appear as holes; people are small black dots, movement is indistinguishable
3000 Houses are blurred, doors are hard to identify, but chimneys on rooftops may still be visible
Everyone has arms. If you ask someone, “How long are your arms?” they might shake their head and say they’ve never measured. If you then ask, “What’s an arm-length ruler?” they’d likely be even more puzzled. That’s because they don’t realize their arms can be used to measure distance.
In fact, an arm-length ruler is simply a pencil (or wooden strip) marked with graduations. But when combined with your arm, it becomes a highly flexible and convenient “instrument” for measuring distance. The graduations on the pencil are marked based on the percentage of your arm’s length (the distance from your eye to the fleshy part of your thumb when your arm is extended forward horizontally) — hence the name “arm-length ruler.” For example, if someone’s arm is 60 cm long, then one graduation on their arm-length ruler would be 6 mm. With an arm-length ruler, as long as you know the size of the target in advance, you can measure the distance. So how is the distance calculated? As mentioned earlier, each graduation on the arm-length ruler represents one percent of your arm’s length. So if the height (or width) of the target occupies one graduation, it also corresponds to one percent of the distance; if it occupies two graduations, it’s two percent, and so on. Therefore, based on the principle of similar triangles and proportional relationships, Distance : Target Height (Interval) = 100 (Arm Length) : Number of Graduations (Arm-Length Ruler). This gives us the formula for calculating distance: Distance = Height (Interval) × 100 ÷ Number of Graduations. For example: if you measure an interval of a telephone pole ahead and find it covers about 5 graduations, and you know the standard interval between telephone poles is 50 meters, then the distance to the pole is: 50 meters × 100 ÷ 5 = 1000 meters.
What if you don’t know the width (or height) of the object? Can you still use the arm-length ruler to measure distance? Yes, but you’ll first need to establish a known distance condition to calculate the desired distance. When using an arm-length ruler to observe various objects, you’ll notice this: the closer you are to an object, the more graduations you’ll measure; conversely, the farther away you are, the fewer graduations you’ll measure. Based on this, you can measure the same target from two different positions to get a larger and a smaller number of graduations, and also measure the distance between the two observation points. With these three known values, you can calculate the distance using the following formula: Distance = Advancing (or Retreating) Distance × (Smaller Graduation ÷ (Larger Graduation – Smaller Graduation)). For example, an engineering unit tasked with building a bridge first sent out scouts to measure the river width. The scout first used the arm-length ruler on the riverbank to measure the interval between two landmarks on the opposite bank as 8 graduations, then moved back 30 meters straight and measured the same interval as 5 graduations. Plugging these values into the formula, the river width is calculated as: 30 × (5 ÷ (8 - 5)) = 30 × (5 ÷ 3) ≈ 50 meters.