L70 LED Lifetime Calculator

Calculate when an LED fixture will reach L70 (70% of initial lumen output) based on IES TM-21 lumen maintenance data. Find replacement schedules, lumen depreciation curves, and project-specific useful life.

LED Fixture Data (TM-21)

Rated L70 Hours

Initial Lumens 4000

Ambient Temperature 25°C

Drive Current vs Rated 100%

Installation

Hours per Day 12

Maintenance Factor (MF)

Target Lumen Threshold 70%

Lifetime Analysis

Rated L70 lifetime—

Temp-adjusted lifetime—

Drive current adjustment—

Effective useful life—

Years at 12h/day—

Lumens at year 5—

Lumens at year 10—

L70 replacement date—

Recommended MF lumen boost—

—

years useful

At hours/day

—

% at yr 5

Lumen output

—

% at yr 10

Lumen output

—

k hours adj.

Effective L70

Lumen Depreciation Curve

—

L90 Point

90% output

—

L80 Point

80% output

—

L70 Point

70% output

Understanding L70 LED lifetime

LEDs rarely burn out suddenly; instead they slowly dim, so lifetime is defined by lumen maintenance. L70 is the point at which an LED has faded to 70% of its initial brightness — widely used as the practical end of useful life for general lighting (L80 and L90 apply where brightness is more critical). Manufacturers estimate L70 from standardized testing (such as LM-80 data projected with TM-21), commonly quoting 25,000, 50,000, or 100,000 hours. The biggest real-world factor is heat: running an LED hot (poor heat sinking or high ambient temperature) accelerates depreciation and shortens L70, while good thermal design and running below maximum current extend it. When comparing fixtures, check both the L70 hours and the temperature they’re rated at.