What do you have to weight to be able to sit in the front seat?

Guidelines vary from state to state. In Washington state it is illegal for anyone under the age of 13 to sit in the front seat of a car if a rear seat is available, regardless of height or weight.

The issue isn't as much as weight (although part of the equation) as it is height. When airbags first started being widely available (first generation airbags) small adults, those around and under five feet tall, were getting killed and maimed by their deployment, along with children.

Second generation airbags deploy slower and some are two stage. Passenger seats on cars built after 2006 have a sensor now to determine the mass of what is in the seat and can decided to not deploy the airbag at all, deploy it at low velocity or high velocity.

To answer your question children under 8 years old, under 4'9" tall and/or under 85 pounds should be in the back seat AND in a booster seat. A good gauge for 4'9" is can they sit normally in a car seat without the booster and have their feet firmly on the floor (not tippy toes). The safest spot is the rear middle seat, if one is available.

For the front seat the rule of thumb is over 13-years old, over five feet tall, and over 100 pounds. No matter what, front seat passengers should always sit as far back as possible.

A rear facing child seat should never, ever, be put in the front seat for any reason.