Generally,

This page will help with standard tread dimensions:
http://www.stair-treads.com/faq/standard-stair-tread-dimensions.html

Radius treads typically refer to the first step in a staircase or a starter step. We offer the following standard sized treads:

Single Turnout Starter Step:
http://www.stair-treads.com/white-oak-single-turnout-starter-step.html

Single Turnout Starter Riser:
http://www.stair-treads.com/stain-grade-single-turnout-starting-step-riser.html

Or you may need a double radius profile:

Double Turnout Starter Step:
http://www.stair-treads.com/white-oak-double-turnout-starting-step.html

Double Turnout Starter Riser:
http://www.stair-treads.com/stain-grade-double-turnout-starting-step-riser.html

We can also make custom radius treads, landing treads, and other profiles. You can reply to this email with your dimensions and we can write a custom quote for you, or you can use our contact form for more info:
http://www.stair-treads.com/contacts/