How to choose a desk or bedside clock
A good desk clock tells the time at a glance, fits the space you have, does not light up the room and wakes you reliably without startling the neighbours. Most buying mistakes come from judging a clock by its photo. This calculator turns your own distance, eyesight and room into numbers that can be compared with a product specification.
Digit height and viewing distance
How easily you read a digit depends on the angle it covers in your field of view, not on its absolute size. A digit that covers about 0.35 degrees is comfortable to read for a person with normal vision; about 0.5 degrees is effortless; and about 0.22 degrees is the smallest size that can still be read with attention. People who do not wear glasses at night, or who have reduced vision, need larger digits, so the calculator multiplies the result by a factor.
Maximum reading distance = digit height ÷ (tan(angle) × factor)
| Distance | Comfortable digit height |
|---|---|
| 3 ft (1 m) – desk | 0.2 in (5 mm) |
| 8 ft (2.5 m) – bed | 0.6 in (15 mm) |
| 15 ft (4.5 m) – across a room | 1.1 in (28 mm) |
Does it fit?
A four-digit display with a colon is roughly four and a half digit heights wide, and the housing adds a bezel and a stand. We estimate the width as 4.6 times the digit height plus 2.4 inches (6 cm). Always compare this with the real product width and leave room for a phone, lamp or glass of water on the nightstand.
Brightness at night
Bright displays can disturb sleep, because even small amounts of light, especially blue and white, suppress melatonin. In a fully dark room an amber or red display of only a few candelas per square metre (nits) is enough. In a dim room with street light you need a little more, and in a bright room you need a lot more, which is why a dimmer with several steps and an auto-brightness sensor are worth paying for. Amber and red are easier on the eyes than blue.
Alarm loudness
To wake reliably, the alarm should be about 15–25 dB louder than the background noise at your ear. Heavy sleepers need more, light sleepers less. Sound falls by 6 dB each time the distance doubles, so a clock across the room must be rated much louder than one on the pillow. For hearing safety, never plan on more than about 85 dB at the ear; gradually rising alarms and vibrating pucks are better ways to wake a deep sleeper.
Rating at 1 ft = target + 20 × log10(distance in ft)
Battery life
Battery runtime equals usable capacity divided by average current. We assume 85% of the rated capacity is usable. A quartz analog movement sips only microamps and runs for years, an LCD with a backlight that is switched on only briefly lasts weeks to months, and an LED display on batteries drains them in days, which is why such clocks should be plugged in and use batteries only as a backup for power cuts.
Buying checklist
- Look for a dimmer or auto-brightness mode and the option to turn the display fully off.
- Choose a battery backup so the time and alarm survive a power cut.
- Radio-controlled models set the time automatically, including daylight saving.
- Check that the snooze button is large and easy to find in the dark.
- Make sure the clock does not rely on a smartphone app to sound the alarm.