The right big and tall winter coat comes down to three questions: what you wear under it, how cold and wet your winter gets, and whether you want a hood. Answer those in order and the choice narrows to one or two styles. Below is the short version of each question, the coat that fits each answer, and how to get the size right when the coat has to go over a sweater or a sport coat.

Question 1: What goes under the coat?

This decides the length and the cut more than anything else.

1. A suit or sport coat. You need a tailored topcoat or overcoat long enough to cover the jacket hem, roughly mid-thigh to knee. Shorter coats leave the sport coat sticking out below.
2. A sweater, flannel or quarter-zip. A car coat, field coat or insulated jacket works. Hip to mid-thigh length is plenty.
3. A bit of both. A city coat in a performance fabric covers a sport coat on Monday and a hoodie on Saturday.

For the suit days: the wool topcoat

The Westport 1989 Wool Blend Top Coat is single-breasted with a notch lapel, a 38-inch length and a scarf pocket inside. Like a sport coat, it is sized by chest in regular, long and extra-long, so buy the same size you wear in a sport coat.

If you want the one coat you will wear for the next twenty winters, the Hart Schaffner Marx Cashmere Top Coat is 100% cashmere, cut to the knee, with pick-stitched details and two interior pockets. It comes in regular and extra-long.

Question 2: How cold and wet does your winter get?

Dry and cold. Wool is the best answer. It blocks wind, holds heat when it is damp and looks right in town. Pair it with a heavier sweater on the coldest days.

Wet, slushy or rainy. Pick a water-resistant or waxed shell. The Peter Millar Maddox City Coat is wind and water-resistant, with a button front over a full zip, and it is long enough to keep a sport coat dry. It also goes in the washing machine, which matters by February.

For a more rugged look, the Westport Black CamoRidge Waxed Cotton Car Coat has a water-repellent waxed shell, snap flap pockets and snap side vents that open when you sit. Car coat length covers your seat without getting in the way when you drive. It runs up to 5XB.

Deep cold. Look for real insulation: down, synthetic fill or shearling. Insulated coats run warmer for their weight, so you can wear a thinner layer underneath and keep the fit clean.

Question 3: Hood or collar?

Tailored topcoats and car coats skip the hood and pair with a scarf. If you walk, shovel or wait on train platforms, a hood earns its place. The Cutter & Buck Rainier Insulated Packable Jacket hides a hood in the back collar, so you get one when the wind picks up and a clean collar the rest of the time. It uses PrimaLoft insulation in a wind and water-resistant quilted shell, packs into its own pocket and runs up to 5XB and 4XT.

Sizing big and tall winter coats over layers

Try any winter coat on over the thickest layer you plan to wear under it: the sweater, the sport coat or both. Then check three things.

Reach. Reach both arms forward as if you are grabbing a steering wheel. The back should not pull and the cuffs should still cover your wrists. If they ride up, try the long, extra-long or tall size.

Close. The coat should button or zip without the front pulling into an X. If it does, go up a size.

Length. A topcoat should cover your sport coat completely. A casual coat should cover your seat. Tall sizes and long lengths add the extra inches.

Tailored topcoats use chest sizes and lengths, the same as sport coats. Casual coats use shirt-style sizes: big sizes from 2XB and tall sizes from LT. Westport's size chart has the measurements for both. For lighter fall layers, our fall jacket guide matches a jacket to the forecast, and the vest guide covers the layer underneath.