How to calculate uptime/downtime %

The formula used in this calculation D/T x 100. D=total downtime in minutes. T=total time in minutes. For example, i need to know the % downtime for last month where the downtime recorded for last month is 4 hrs.

(4 x 60) / ((30 x 24)  x 60) x 100 so we get the downtime. so (240 / 43200) x 100 should get 0.555555556. Uptime will be 100 – 0.555555556=99.444444444%.