A fist at arm's length covers ten degrees of sky; three fingers cover five; a pinky covers one. It's the improvised ruler amateur astronomers have long used to gauge how far one star sits from another. Turned on your own eye, that same ruler finds something unexpected: a real gap in your visual field, wider than half a fist.
The instrument below brings the classic ophthalmology test onto a screen, and puts it on that same ruler, in degrees. Close one eye, fix your gaze on the cross, and slide the red dot toward the edge until it vanishes — then keep going until it reappears. The gap between those two moments is your own blind spot's angle.
Instrument
Find your blind spot
- Hold the screen at a comfortable reading distance — or move closer if you're on a phone.
- Close your left eye.
- Stare fixedly at the cross on the left. Don't shift your gaze to the dot, even as it nears the edge.
- With your right eye, use the slider below to move the red dot rightward.
default, unadjusted
Vanishing point: —
Reappearing point: —
Want the exact number? Calibrate with a card (30 seconds)
Hold a real card (bank or ID) flat against the screen and match the rectangle below to its edges.
Measure with a ruler, or estimate: an open hand span is about 18-20 centimeters, an outstretched arm is about 55-60 centimeters.
Why you never see it
The gap exists because, exactly where the optic nerve leaves the eye, the retina has no photoreceptors. You never notice it for two reasons: each eye's blind spot sits in a different part of the visual field, so one eye covers the other's gap, and the brain fills the missing patch in with information from around it even with one eye closed — a phenomenon called perceptual filling-in, demonstrated experimentally in 1991 by Vilayanur Ramachandran and Richard Gregory. The full anatomical mechanism, with a retinal map and a separate centimeter calculator at reading distance, is covered in depth in "Five Centimetres of Nothing at 60 Centimetres", another page on this site; here the focus stays on the angle, not the anatomy.
How big it actually is
The first precise maps of the blind spot came from ophthalmologist Mansour Armaly, who in 1969 measured a blind spot centered about 15 degrees temporal to fixation, spanning 6 to 8 degrees. In 2021, using a finer computerized method, Xiao Ling and colleagues confirmed the figures: across their 12 participants, the blind spot averaged 6.28 degrees wide (range 5.39–7.44) and 7.02 degrees tall (range 5.79–8.14), centered at 16.00 degrees temporal.
Put on the astronomer's ruler, the figure surprises: the moon or the sun covers only about 0.5 degrees of sky — an average blind spot is more than twelve times wider. Three fingers at arm's length cover about 5 degrees, and a full fist covers about 10: the blind spot already clears "three fingers" and reaches 63% of a full fist in width, 70% in height.
| Reference on the sky's ruler | Angle |
|---|---|
| Moon or sun, apparent diameter | 0.5° |
| Pinky finger at arm's length | ≈1° |
| Three fingers at arm's length | ≈5° |
| Blind spot, average width (Ling et al., 2021) | 6.28° |
| Blind spot, average height (Ling et al., 2021) | 7.02° |
| Fist at arm's length | ≈10° |
What this test doesn't show. The test above depends on your calibration — how well you matched the card on screen and how accurately you measured the eye-to-screen distance. Real variation exists too: in the 2021 study, blind-spot width ranged from 5.39 to 7.44 degrees across just 12 healthy participants. This is not a diagnostic test — a much larger or displaced blind spot can have medical causes, but that's established only with a clinical perimeter, not a screen at home.
Method and sources
The blind-spot dimensions and location cited above come from two independent sources: Mansour Armaly, "The size and location of the normal blind spot", Archives of Ophthalmology, 1969 — the classic perimetry study; and Xiao Ling, Edward Silson, Robert McIntosh, and Nicholas Swindale, "Did you see it? A Python tool for psychophysical assessment of the human blind spot", PLOS ONE, 2021, using a computerized method with 12 participants. The filling-in explanation draws on Vilayanur Ramachandran and Richard Gregory, "Perceptual filling-in of artificially induced scotomas in human vision," Nature, 1991.
The angle-to-millimeter conversion uses the eye's standard nodal distance of 17 millimeters (the reduced eye), a constant from clinical optics. Your own angle, computed by the instrument above, rests entirely on the geometry you enter: the calibration card's width (85.60 millimeters, the ISO/IEC 7810 ID-1 standard) and the distance you declare. The "fist = 10 degrees," "three fingers = 5 degrees," and "pinky = 1 degree" references are the rule of thumb amateur astronomers use for quick sky estimates with no instrument.
For the full anatomical mechanism of the blind spot — the retinal map, the role of binocular vision, and a separate calculator for its size in centimeters at reading distance — see also "Five Centimetres of Nothing at 60 Centimetres", a page taking a different angle on the same subject.