The fireplace grate is the unsung workhorse of your home fireplace system. It doesn't get put on display, like a fireplace tool set. It doesn't frame the fireplace like a set of glass doors. It doesn't draw attention to itself at all. The fireplace grate simply holds the wood in place so that you can set fire to it and enjoy the warmth and the view.
A basic fireplace grate consists of a metal framework that holds the logs in place. The metal is usually cast iron or steel and should be sturdy enough to take a lot of abuse. It will have four or more feet to hold the grate up from the floor of the firebox. Often these feet are simply extensions of the metal framework that are bent downward.
There are several things to consider when purchasing a new fireplace grate.
Size may be the most important aspect to consider in a fireplace grate. You want it to be able to fit nicely into your fireplace without scraping against the walls or, worse, putting the fire too near the top.
To find out what size grate you need, you need to know the measurements of the inside of your firebox. If you don't have those specifications handy, you can take the measurements yourself with a tape measure. You want to know the measurements for the width of the opening into the firebox (how much space is there between the fully opened glass doors?), the depth of the fireplace (how far back does it go with the doors fully closed), and the height. Don't measure the height from the center where the ceiling rises up into the flue. Rather, measure the height along one of the sides.
With those parameters in mind, feel free to buy the largest grate that will fit while still leaving you a few inches clearance on all four sides of the grate.
You also want to make sure you have enough clearance at the top. Allow for a generous amount of space between the top of the wood pile on the grate and the bottom of the damper. You don't want flames actually shooting up into the chimney. At the same time, you do want the legs of the grate to be reasonably tall. Four inches should be plenty. The purpose of the grate will be frustrated if you don't get a good air flow going in under the grate and coming up through the logs to fuel the fire.
If you have a "zero clearance" fireplace, make sure you get a grate that is rated for use with that appliance.
Fireplace grates take a lot of abuse. Over time, they can fall apart. Some manufacturers will rate their grates for life, others for three years. Make sure that whatever you get is sturdy and that any welds are strong. It's no fun replacing one of these things.
Fireplace grates can cost anywhere from $30 to $150.