Everyone knows that an umbrella keeps your head dry and your shins wet. This worksheet puts a number on it. It takes two published inputs, how fast raindrops fall and how fast the wind blows at each Beaufort force, adds a canopy size and some assumptions about you, and gives you the depth below the canopy edge that stays dry. The inputs are cited. The geometry and the table are our own arithmetic, not a test, and the assumptions are listed so you can change them. xzvzx and Calm Eden share an owner.
The two inputs
| Input | Figure | Source |
|---|---|---|
| Drizzle, 0.5 mm drops | About 2 m/s fall speed | Wikipedia, Rain, as read on 6 November 2026 |
| Large drops, 5 mm | About 9 m/s | Wikipedia, Rain |
| A 3 mm drop | About 8 m/s | Wikipedia, Raindrop |
| Wind by Beaufort force | Mean speed per force, in metres per second | Met Office, Beaufort scale |
The Met Office table gives a mean of about 3 m/s for force 2 (light breeze), 5 for force 3 (gentle breeze), 7 for force 4 (moderate breeze), 10 for force 5 (fresh breeze) and 12 for force 6 (strong breeze). It describes the scale as an empirical measure of wind intensity based on observed sea conditions, so treat it as the open-water figure. A built-up street is usually calmer than a forecast.
The formula
Rain that slips in at the upwind edge of the canopy travels sideways as it falls. For every metre it drops, it moves w divided by v metres across, where w is the wind speed and v is the drop’s fall speed. You stay dry as long as that sideways shift has not yet reached the edge of your body. So:
Dry depth below the canopy edge = (canopy radius minus half your shoulder width) x v / w
All in metres. We assume the drops move at the wind speed, which is generous to the rain, and that the canopy is upright and you stand under its centre.
The table
Assumptions: shoulders 50 cm across, so half is 25 cm. Canopy rim 190 cm above the ground, which puts your shoulders at roughly 45 cm below it if you are about 175 cm tall. Dry depth in centimetres below the rim, rounded.
| Beaufort force (mean wind) | 105 cm canopy, drizzle | 105 cm canopy, 3 mm drops | 140 cm canopy, drizzle | 140 cm canopy, 3 mm drops |
|---|---|---|---|---|
| 2, light breeze (3 m/s) | 18 | 73 | 30 | 120 |
| 3, gentle breeze (5 m/s) | 11 | 44 | 18 | 72 |
| 4, moderate breeze (7 m/s) | 8 | 31 | 13 | 51 |
| 5, fresh breeze (10 m/s) | 6 | 22 | 9 | 36 |
| 6, strong breeze (12 m/s) | 5 | 18 | 8 | 30 |
The 105 cm column is the canopy size of Calm Eden’s Haven Umbrella, which opens to about 105 cm. The 140 cm column is an illustrative wider canopy, not a named product.
Reading it
- Shoulders are about 45 cm below the rim in this setup. A figure under 45 means the shoulders are getting wet. Under a 105 cm canopy and 3 mm drops that starts at force 3, where the figure is 44. With drizzle the shoulders are wet from force 2.
- Drizzle is the sneaky one. Slow drops get carried further. The same breeze that leaves you dry in a downpour dampens your sleeves in a mist.
- A bigger canopy helps, but not as the width suggests. Going from 105 to 140 cm is a 33 percent wider canopy and a 64 percent deeper dry zone in this model, because what counts is the gap between canopy radius and your own half-width.
- Nothing here keeps your shins dry. Even the best cell, 120 cm in a force 2 breeze, leaves you dry only down to about 70 cm above the ground. Slanting rain gets legs and feet under any handheld umbrella.
What to do with the result
- Dry depth over 45 cm: an umbrella is enough for the upper body. Keep a waterproof for your lower half if you will be out long.
- Dry depth 15 to 45 cm: the umbrella is a head-and-hair tool. Wear a hooded jacket and tilt the canopy into the wind. See our umbrella or rain jacket guide for the choice.
- Dry depth under 15 cm: you are mostly relying on the clothes. Use the umbrella only for your face and glasses, and check our umbrella wind speed chart before you open it in force 6.
For the rain half of the forecast, read our rain intensity chart next. If you are choosing a size, the umbrella size worksheet starts from your bag and your day instead. The sun-side equivalent of this sheet is where an umbrella’s shade lands.
What the sheet does not know
- It does not know your street. Buildings, trees and corners break the wind, and the Met Office figure is a sea-based mean, not a gust.
- It treats raindrops as riding the wind at full speed, which overstates the slant if real drops lag the wind a little. In that case the true dry depth would be somewhat larger.
- It ignores tilt, walking speed and where the canopy sits over you. Walking into the wind adds to the slant; walking with it takes some away.
- Wikipedia is a general reference, not a measurement paper, and the fall speeds are for sea level and still air.
Where the Haven Umbrella fits
Calm Eden’s Haven Umbrella is a single-adult compact. Its listed figures are 30 carbon fibre ribs, an opening of about 105 cm, a fold to 40 cm / 15.75 in and a weight near 390 g, with a one-press open and close. It is a head-and-shoulders umbrella and the worksheet above is why we say so; for the long form of that argument see Calm Eden’s full-body umbrella guide. Every order includes a Haven Cover and the Wellness Journal (digital); Buy 2 Get 1 Free adds the UV Charm and the Rain Poncho, and Buy 3 Get 2 Free also adds the Haven SPF 50+ Silk Square Scarf.
Sources: Wikipedia, Rain and Raindrop; Met Office, Beaufort wind force scale. All read on 6 November 2026.
Common questions
Why do my legs get wet under an umbrella?
Because rain arrives at a slant once there is any wind. Wikipedia gives a fall speed of about 2 m/s for drizzle and about 9 m/s for large drops, so a breeze of a few metres a second tilts the path enough to carry drops under the canopy edge. The sheet turns that into a distance.
How much does a bigger canopy help?
In this model the dry depth is proportional to the canopy radius minus half your shoulder width. Going from a 105 cm to a 140 cm canopy takes that gap from 27.5 cm to 45 cm, so the dry depth grows by about 64 percent, not by the 33 percent the width suggests.
Does drizzle or heavy rain get under the umbrella more easily?
Drizzle. Slow drops spend longer in the air, so the wind carries them further sideways before they land. At force 4 the sheet gives 8 cm of dry depth for drizzle against 31 cm for 3 mm drops under a 105 cm canopy.
Where do the wind speeds come from?
From the Met Office Beaufort table, which lists a mean wind speed in metres per second for each force: about 3 for force 2, 5 for force 3, 7 for force 4, 10 for force 5 and 12 for force 6. The Met Office describes the scale as a measure based on observed sea conditions, so a street can be calmer.
Does tilting the umbrella change the answer?
Yes, in your favour. Tilting the canopy into the wind moves the dry zone downwind. The sheet assumes an upright canopy, which is the worst case, and does not put a number on tilt.