New to boots? Its not as confusing as you think!
First time boot users coming out of shoes please note:
If you are transitioning a long term shod horse into boots it is best to wait until the 2nd trim cycle before you measure and purchase your boots. Just remove the shoes and leave the horse's hooves to settle down for a few weeks. Do not do any trimming at first just remove any flares near the ground surface that might break away and leave the sole alone till the follow up trim.
Be considerate of your horse. He is going through a lot of changes and may be tender at this time. If you are planning riding him and he is tender then it may be best to wait till you have your new boots for hoof protection.
For good boot fit your trim cycles should be no longer than 4 weeks apart. Please use a correct barefoot trim to set up the foot. Do not set up your foot in a flat style as if you were going to apply a shoe after your trim. DVD instructional information on barefoot trimming can be sourced from www.barehoofcare.com if you need it.
Now to the business of choice:
Remember the over-rding consideration for boot choice should be correct fit. ALWAYS choose the boot style with a sizing that is closest to your horse's hoof measurements.
Note the chart is in miles not kilometers.

2. What shape hoof does your horse have?
Boot styles come in various shapes.
Do not just trace a hoof shape on paper and use those measurements against the sizing charts. Measure the actual hoof itself, then look for the sizing closest to your horse's measurements.
Boot sizing allows for a growth period between trims so be sure to do this after a fresh trim.

When measuring it is important that you take a good look at your horse's sole shape as well as the shape of his entire hoof capsule (walls).
Well trimmed feet should not have excessive flares. If you are concerned about your horse's hoof shape consult with your barefoot trimmer prior to purchase.

3. What type of fastening system can you manage?
Because boots have different fastening systems it is important to consider your own personal capabilities. Easycare Inc. have developed a "degree of difficulty" scale to help you assess how hard each boot is to fasten.

4. Other Considerations:
Does your horse have miss-matched feet - they may have a clubbed hoof! (High/low hoof syndrome).
There are a large percentage of club footed horses in the domestic horse population. There are many theories as to why this type of hoof form occurs but it brings with it two very different hoof shapes.
One front hoof will be low and flat (sometimes with under run heels and a long toe) whilst the other will be smaller and more contracted at the heel (clubbed) with different angle on the front wall and a steep heel.
Because of this difference it is important to choose a boot style that allows you to buy two different sized boots and one that allows you to also pad one of the boots if necessary to adjust your fit. Some boots are sold as pairs and some as single boots so this may affect your choice.
Easycare Down Under's Equine Myofunctional Therapist would like us to also mention that any horse with high/low hoof syndrome will also have upper shoulder muscular asymmetries.
Because these horses habitually prefer to load the lower more splayed hoof and graze with the more upright one placed behind, they develop one large shoulder/wither area and on the opposite side will have muscle atrophy (wastage). They will also have a favourite side, and prefer this for their canter leads and trot diagonals etc.
Because tree-ed saddles are made symmetrical, this difference in their upper shoulders will often cause saddle fit issues, as when the muscles that sit over the scapular (on the larger shoulder) pass under the front of your saddle tree with every single step, they may be crushed and damaged. This also will cause the saddle to see saw across the horse's spinal processes at the cantel causing pressure and friction at the back of the saddle. For horses with muscular asymmetries it is often best to consider a treeless saddle.