Although drywall is relatively inexpensive, it's also VERY heavy.
Accurate estimating is critical not only do you want to spend
the extra money for sheets of drywall that you don't need, you
also don't want to have extra around that you'll need to return.
In my case, since I needed to cut each sheet to fit it into the
basement (see
drywall and insulation delivery), that would not have been
an option.
Chances are you'll be estimating amounts for two types of drywall
one for your 'regular' walls (usually ½" or 5/8") and one amount
for the sheets of fire code drywall (at least 5/8") you'll need.

I needed fire code drywall on the walls near the furnace, and
also under the stairs. This is required because if you have a
fire in the basement, fire coded drywall on the bottom of the
steps will give you and your family more time to get out! Take
a look at the diagram below to see where I used regular drywall
(yellow wall line) and fire code drywall (denoted by red lines).
Drywall
estimating tips
 Measure the total length for all of your walls. Remember that
for some walls, you'll need drywall on both sides of the wall.
 Be sure to measure and total all walls where fire code drywall
will be used independently of those where you will use standard
drywall.
 DO NOT SUBTRACT ANYTHING for door openings
and window openings. (For example if you have a 3' wide door
opening, that still counts a 3 feet of drywall.)
 Take the total number of feet you measure and add 10%  15%
to get the total number of lineal feet required. Whether you
add 10% or 15% depends on:
 How confident you are that you won't ruin sheets of drywall
during the cutting/hanging process.
 Determining whether it would be more difficult to return
the extra sheets (if that's even an option), or would it
be more difficult to buy a few more sheets after the fact
if you're running short.
 Assuming your basement ceiling will not be higher than 8',
and you're using 4x8 sheets of drywall, divide the total number
of lineal feet of drywall by 4. This is the total number of
sheets you should buy. (Round up any leftover amount.) If you're
hanging two layers of drywall (for extra soundproofing), multiply
your final number by two.
 If you've done this correctly, you should end up with just
about the right amount of drywall give or take a few sheets.
Drywall
estimating example
Take a look at the example below. For this example we'll estimate
drywall needed to finish the basement office.
Based on this example we have:
 (3) 12' walls
REMEMBER: The wall with the door needs to be
finished on both sides, AND we do NOT subtract anything for
the 32" doorway.
 (1) 10' wall
 (1) 8' wall
Now, we add the lengths of all the walls: 12+12+12+10+8 = 54'
of wall space. Adding 10% (about 6' rounding up) to this total
gives us 60 feet of wall to finish.
Assuming 4x8 sheets of drywall, and 8' ceilings, this means we'll
need 15 sheets of drywall to finish the office space.
See how easy that is!
Additional drywall resources
How to hang drywall  Learn
the ins and outs of hanging drywall for your basement remodeling
project.
How to cut around outlets
 Learn to cut around outlets, switches, and other openings.
Drywall tools  Here are
the things you'll need to complete your basement remodeling project.
