Monostable mode lets you use the 555 timer chip as a single-event timer. The time interval for a 555 monostable circuit is a measure of how long the output stays high when it's triggered in an electronic gadget of some kind. To calculate the time interval, just use this formula:

T = 1.1 R C

Here, T is the time interval in seconds, R is the resistance of R1 in ohms, and C is the capacitance of C1 in farads.

For example, suppose R1 is 500 kΩ, and C1 is 10 μF. Then, you would calculate the time interval like this:

T = 1.1 500,000 Ω 0.00001 F
T = 5.5 s

Thus, the circuit will stay on for 5.5 seconds after it's triggered.

When you make this calculation, it’s imperative that you use the correct number of zeros for both the resistance and the capacitance.

Resistance Capacitance
1 kÙ = 1,000 Ù 0.01 ìF = 0.00000001F
10 kÙ = 10,000 Ù 0.1 ìF = 0.0000001F
100 kÙ = 100,000 Ù 1 ìF = 0.000001F
1M Ù = 1,000,000 Ù 10 ìF = 0.00001F
10M Ù = 10,000,000 Ù 100 ìF = 0.0001F
100M Ù = 100,000,000 Ù 1,000 ìF = 0.001F