With data:

Area of 0 to 500
Area of 501 to 1000
Area of 1001 to 1500
Area of 1501 to 2000
Area of 2001 to 2500
Area of 2501 to 3000

In A1:A6 I use the formula:

=SUMPRODUCT(--VALUE(MID(A1:A6,FIND("to",A1:A6)+2,LEN(A1:A6)-FIND("to",A1:A6))))

to sum up the MAXIMUM Limit in entries resulting to:
10,500

However, how to have the same formula be applied and working correctly if
there is a blank cell in A1:A6?

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Thanx in advance,
Best Regards,

Faraz