Tau Symbol (τ): Torque, Time Constant and LaTeX
July 15, 2026
The tau symbol is τ, the nineteenth letter of the Greek alphabet. It means torque in mechanics, a time constant in circuits, and shear stress in materials. Copy it below, or jump to how to type it.
| Character | Unicode | HTML entity | LaTeX | Windows alt code |
|---|---|---|---|---|
| τ lowercase tau | U+03C4 | τ | \tau | Alt + 231 |
| Τ uppercase tau | U+03A4 | Τ | T | — |
What the symbol means
| Where | Meaning |
|---|---|
| Mechanics | Torque — the rotational equivalent of force |
| Electronics | Time constant of an RC or RL circuit, in seconds |
| Materials | Shear stress, force parallel to a surface |
| Relativity | Proper time, measured in an object's own frame |
| Statistics | Kendall's tau, a rank correlation coefficient |
| Particle physics | The tau lepton, a heavier cousin of the electron |
| Maths | Sometimes 2π, in the 'tau over pi' convention |
τ as torque
Torque is the twisting effect of a force applied at a distance from a pivot. Only the perpendicular component does any turning, which is where the sine comes from — push straight at the hinge and nothing rotates.
Uppercase tau (Τ) is visually identical to a Latin T, so it is essentially never used. Torque uses the lowercase letter; a capital T in physics almost always means period or temperature.
τ as a time constant
In an RC circuit, τ is the product of resistance and capacitance, and it has units of seconds. After one time constant the quantity has fallen to about 37% of its starting value; after five, to under 1% — which is the usual rule of thumb for “fully settled”.
τ as shear stress
Normal stress σ acts perpendicular to a surface; shear stress τ acts along it. The two together describe the full state of stress at a point, and the distinction matters because most materials fail differently under each.
How to type it
Windows
- τ: hold
Altand type231on the numeric keypad - In Word, type
03C4and pressAlt + X - Elsewhere, press
Windows + .and search “tau”
Mac
- Open Character Viewer with
Control + Command + Spaceand search “tau” - Or enable the Greek keyboard, where τ sits on the
tkey
HTML
τorτfor τ
In LaTeX
\tau in math mode. For torque as a vector, use \boldsymbol rather than \mathbf — the latter does not embolden Greek letters in most fonts.
Frequently asked questions
What does τ mean in physics?
Most often torque, the rotational equivalent of force. In circuits it is the time constant; in relativity, proper time.
What is the time constant τ?
The time for a quantity to fall to about 37% of its initial value in exponential decay. For an RC circuit it equals resistance times capacitance.
Is τ the same as 2π?
In one proposed convention, yes — the 'tau movement' argues 2π is the more natural circle constant. It is not standard, so define it before using it that way.
See also the omega symbol, alpha symbol and theta symbol.
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