Getting the perfect fit for your pet, you'll need a standard tape measure and patience. Your pet dog will think it is a new game you've made up so approximations must be more rather than less.
While lying down...

Step 1 Get your pup/dog to lie still for a while, measure your pet from nose to middle of the back then from that point to it's rump. Measuring from nose to rump is usually difficult with an excitable dog, breaking measurements to two points get better results for length.

Step 2 When you've got the length, add stretching allowance about 3 in – 4 in (minimum). You can add a few more inches if your pet dog has a sleeping quirk that requires space.

 Step 3 Lay your dog on its side, measure your dogs height from forehead down to its paws.

Another technique in measuring your pet dog for its bed size is to measure it while it is standing. This works for fidgety dogs or pets that are curious abut the tape measure.

Step 1 Mark the the start of the tape measure with your thumb nail. Some make the mistake of including the metal part as part of the measuring. Start measurement at the nose of your pet. Make sure it is looking straight ahead and not up.

Step 2 Mark the measurement with your nail at your dogs' back. Take not of the length measured so you can pick up where you left off just in case you let go.

Step 3 Continue measuring from the length last measured and proceed until the tip of the tail.

Step 4 Measure from its head downwards to its paws.

So you've got two measurements, one for the length of your pet and the other the height of your pet. Add 5in-7in for stretching allowance on both measurements.

Keep these figures in mind or in your record, and your good to go.