I have a 6 hour layover at Dulles. Is there a way to get to the DC area and back in that amount of time? Just want to do some walking around and see the buildings>

Your best bet is to visit the new branch of the Smithsonian Air and Space Museum. https://airandspace.si.edu/visit/udvar-hazy-center/
This new branch is very close to Dulles in Chantilly.
If you ever want to go into downtown Washington DC, fly into Reagan National and take the Metro into the city.

Does the "6 hour layover" count the time you need to be back to the airport to catch your next flight? Or we need to subtract approx 1.5-2.5hrs to accommodate that?

Flight arrives into Dulles at 3:30 p.m. and leaves at 10 p.m.

IAD is more than 20 miles each way from downtown DC. You'd need a fast cab both ways. I would opt for the Udvar as a substitute as was suggested.

So closer to 3.5 to 4 hours actually free.

By the time you land then deplane likely will be at least 4:00pm & depending if international or domestic onward flight, you'll need to be back at the airport between 7:00-8:30 pm.

The train from Dullas to DC takes minimum of 1 hour, 32 minutes, but usually more - 1 hour, 40 minutes +, so you must allow 3 & 1/2 to 3 & 3/4 hours RT. Trains do not run back to back, so you may have a wait for the train at either end, adding a bit more time.

If all domestic flights, allow 10-15 minutes for getting off your first flight and an hour for security and boarding your next flight. Total of 5 hours. If everything went like clockwork, (your arriving flight isn't late by even a few minutes), then you might have an hour in DC, but if one thing goes askew, you have a lot to lose. Big hassle for little reward and lots of stress.

If you arrive or depart on an international flight, you must add time for going through passport control on arrival or two hours prior for departure, so it would not be possible.

Don't even think of getting a taxi. You never know about traffic and it can be crazy.

I do not recommend trying to get into the city, you don't have enough time. Udvar-Hazy is a good alternative, if it interests you.

Sorry, Did not mean to post yet. I meant to include, the metro train. goes only part way out from DC. It is a bus/train combo with bus part from Dullas then connecting with metro train to DC. Not a fun trip.

Anyway, my final advice is the same. Too much hassle for too little reward and too many possibilities of something going wrong.

Same advice for taxi or car. It can be clear or have unbearable traffic.

After May 27 the Udvar Hazy will be closing at 6:30PM. Until then it will be closing at 5:30PM. Plan on 15-20 minutes back to Dulles probably by taxi.

Thank you all for your suggestions and replies!