Help with DBgrid

Please help with following:

I have a DBgrid which carries information regarding dangerous goods to be transported by air

I have two columns which have different data - One carries max weight/volume per item allowed on passenger aircraft
and the another max weight/volume per item allowed on cargo aircraft.

A third column is used to calculate the total by multiplying quantity with mass.

At the start the Passenger column is vissible and calculation happens automatically as data is entered.
If however the maximum for items on passenger aircraft exeeds the allowed maximum I need then to switch to cargo aircraft.

I do this by hiding the passenger column and make the cargo one vissible.

I then need to re calculate the third field based on the new column data.

The only way I can manage this is by selecting line by line in the dbgrid and update and click the update command button.

Please can sombody help with an easier way like when I click the update command button all the lines get recalculated at once

Hope this makes sense to somone