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Source: D&D Wiki

Any portable object can be used as a weapon in a pinch. In most cases, an object can be wielded either as a melee weapon or a ranged weapon.

A character takes a –4 penalty on his or her attack roll when wielding or throwing an improvised weapon. An improvised weapon is not considered simple, archaic, or exotic, so weapon proficiency feats cannot offset the –4 penalty.

Table: Improvised Weapon Damage by Size

Object Size	Examples	Damage
Diminutive	Ashtray, CD disk case, crystal paperweight	1
Tiny	Fist-sized rock, mug, screwdriver, softball, flashlight, wrench	1d2
Small	Bottle, drill, fire extinguisher, flower pot, helmet, metal hubcap, vase	1d3
Medium	Bar stool, brick, briefcase, bowling ball, garbage can lid, hockey stick, nail gun	1d4
Large	Empty garbage can, guitar, computer monitor, office chair, tire iron	1d6
Huge	10-foot ladder, mailbox, oil barrel, park bench, sawhorse	1d8
Gargantuan	Desk, dumpster, file cabinet, large sofa, soda machine	2d6
Colossal	Junked vehicle, stoplight, telephone pole	2d8

A character can effectively wield or throw an object of his or her size category or smaller using one hand. A character can effectively wield or throw an object one size category larger than him or herself using two hands. An improvised thrown weapon has a range increment of 10 feet. Increase the range increment for creatures of Large size or larger as follows: Large 15 feet, Huge 30 feet, Gargantuan 60 feet, Colossal 120 feet.

Damage
Improvised weapons deal lethal damage based on their size, although the GM may adjust the damage of an object that is especially light or heavy for its size. The wielder’s Strength modifier applies only to damage from Tiny or larger improvised weapons; do not apply the wielder’s Strength modifier to damage from Diminutive objects. Table: Improvised Weapon Damage by Size gives the damage for improvised weapons of varying size. Improvised weapons threaten a critical hit on a natural roll of 20. Improvised weapons of Fine size deal no damage.

Unlike real weapons, improvised weapons are not designed to absorb damage. They tend to shatter, bend, crumple, or fall apart after a few blows. An improvised weapon has a 50% chance of breaking each time it deals damage or, in the case of thrown objects, strikes a surface (such as a wall) or an object larger than itself.