Is this from biorad... I can't just look at the comb, how thick are you going to pour the agarose, so I'm going to take the square gel tanks that come with these and assume that I'm pouring 1/4 of the gel (i.e., the smallest square gel tanks, with half of the comb, 6 holes for the 13-tooth, and 8 holes for the 18-tooth, right?), and that the agarose system is 20mL, no matter the concentration. 25-tooth is probably about 5ul. 18-tooth is about 20ul. 13-tooth is about 35ul, or so. The 13-tooth is about 35ul. The largest one, the marker channel is arbitrary, and should hold about 100ul in the hole. But to be honest, I've never used this comb... Actually, if you have a large volume of samples, pour the gel thicker, otherwise just thin it out. When I do my digests, I use a 13-well comb with a small compartment, and I can pour almost 30mL of gel (it will diffuse out a bit...) , and then can run more than 50uL...