**Length of Backup Time Explained **

A common question about backup is "how long should my backup take?" Well, we have a mathematical formula for you that will give you an *approximate* time frame of how long your backup should take you.

Don't want to figure this out by hand? Then use our online **Length of Backup Time Calculator**.

Manual Method

Explanation:

Take the size of your backup in megabytes, divide it by the expected average throughput of your backup device measured in megabytes per second, divide that number by 60 and divide it by 60 again. This is the time, in hours, that it should take for your backup, but we're not done yet. The average tape drive takes about six minutes for a tape to be rewound, ejected, put back into the slot in which it came from, then a new tape to be loaded and come online. So, if you had a backup that spanned 4 tapes, you're looking at an additional 24 minutes during the backup process.** **Once you have this number, multiply it by two and that's the total amount of time you can expect for a backup to complete to tape, including the verify pass.

For example:

Step 1:

Calculate BRU's overhead:

Size of backup in MB X 1.15 = Total size of backup in MB with BRU's overhead

This will give you the size of your archive in megabytes with BRU's overhead at 15% factored into the total.

Step 2:

Size of backup in MB ÷ Average speed of drive in MB/sec ÷ 60 ÷ 60 + (6 x number of tape changes)

So if I have a backup that's 1TB and its going to an LTO-3 tape drive that averages 40MB/sec that spans 3 tapes, the total backup time is:

1,120,000 ÷40 ÷ 60 ÷ 60 = 7.777

This is the total time for the backup, about 8 hours, but the verify still needs to run, so multiply that number by two and the total from this would be about 16 hours because:

(1,120,000 ÷ 40 ÷ 60 ÷ 60) x 2 = 15.555 hours.

Step 3:

Now we need to add in the tape changes. Since a 1TB archive would require 3 LTO-3 tapes, then we have to account for an additional 18 minutes (6x3=18.) To do this correctly, we have to change the minutes to hundreths of an hour. Use this table to assist you:

MinutesHundredthsMinutesHundredthsMinutesHundredthsMinutesHundredths1 .02 16 .27 31 .52 46 .77 2 .03 17 .28 32 .53 47 .78 3 .05 18 .30 33 .55 48 .80 4 .07 19 .32 34 .57 49 .82 5 .08 20 .33 35 .58 50 .83 6 .10 21 .35 36 .60 51 .85 7 .12 22 .37 37 .62 52 .87 8 .13 23 .38 38 .63 53.88 9 .15 24 .40 39 .65 54 .90 10 .17 25 .42 40 .67 55 .92 11 .18 26 .43 41 .68 56 .93 12 .20 27 .45 42 .70 57 .95 13 .22 28 .47 43 .72 58 .97 14 .23 29 .48 44 .73 59 .98 15 .25 30 .50 45 .75 0 .00

So we need to add .30 to this total.

15.555 + .30 = 15.855 hours

Using the table above and our math, we can say that a 1TB archive, written at 40 MB/Sec to an LTO-3 tape drive should take approximately 15 hours 51 minutes, or simply put, 16 hours.

**Easy Method**

Don't want to figure this out by hand? Then use our online **Length of Backup Time Calculator**.

Please note that BRU does incur a tape overhead of 8-15%. What does this mean? Use our **Backup Size Calculator** to determine how much this adds to your backup.