Larger tires on a road bike

Well, it depends a lot on the specific model of bike, and likely even on the exact frame size. You can probably eyeball it to get a good idea. Remember that larger tires are both wider and stick out more from the wheel. On a road bike, odds are high that you can put a slightly larger (say, go from 25mm to 30mm) tire but not a hugely larger tire. If you want to make a big jump you probably also want to get wheels with a wider rim. Note that for off-road tires, the knobs aren't usually counted in the mm size but definitely do affect whether or not the tires fit on your bike.

Look for:

1. Front and rear: Brake clearance — can a larger tire fit in the brakes? If this is the only issue, you can get different brakes
2. Rear: Stay width — will a wider tire rub on any of the stays (rear)?
3. Rear: Stay bridge clearance* — will a larger tire hit the bridge between seat stays or chain stays?
4. Rear: Seat tube clearance — many bikes have a geometry where too large of a wheel/tire will hit the seat tube above the cranks
5. Front: Fork clearance — how much room is there under the fork crown? (height and width)
6. Front: Down tube clearance — how much extra room before the front tire hits the downtube?

If your bike has 700C wheels, you could convert to 650B. The wheel radius is only 25mm less, so most things work the same, but you have that much more room for tires and fenders.

You'll probably need to replace your brakes with long-reach brakes.

For our small expedition with our bicycles in Patagonia we invested into slightly larger tires and more durable ones (we choose the Schwalbe Marathon XR in 700C, the widest our bike frame could accomodate).

It did quite a change, you hear more the tire noises, it helps better in muddy terrain and feels more confortable in gravel roads or in the fields. However, it's not really good in sandy parts or when the gravels are too big or too loose.

Overall it was a good and needed change, it would have been much less confortable to ride down in Patagonia with out those wider tires.

If you decide to change yours, follow freiheit advice or go pay a visit to a local bicycle shop. It's not always possible to put different tires because of the rim, brakes or frame.

I have a Norco Search 2017 Sora and actually has 700x38C tires (B'Twin tires). It came originally with Schwalbe Road Cruiser 700x32c K-Guard. So, it depends what are the height and width of the fork.

In the eight years since this question was written, road-style bikes with fatter, grippier tyres for off-road use have become pretty common. They're usually called "gravel bikes" or "adventure bikes". Plenty of companies are making bikes of this type: essentially, a relaxed-geometry road-ish frame with enough tyre clearance for something like 40mm tyres for use on ground that's, say, as rough as a hiking trail but not so rough that you'd need a mountain bike.