First Impressions:
Before I could get a proper look at your place I had a pop up covering the page and a redirect working in the background. Through the years, many people have learned to distrust and avoid sites that do this kind of thing. I think many of your potential users don't even stay to check what your site has to offer.
It is important that nothing in your site takes away the control from the user like that. Many will flee afraid a trojan or something harmful loads regardless how secure the adverts might be. Users with ad-blocker won't feel any of this, but that just means you are less likely to have people disabling it to help you.
The huge pop up with another advert is also a demotivating element. I find it more bearable but not something I'd feel okay to put on with often.
Pop-ups are annoying even when they come from the same site so if you can do without them it is even better. I would recommend that you only use pop-ups for things related to your site and only use them to convey important information. The less of the screen they take the better.
Advertisement scares aside, your site looks nice. It is simple to understand and the base seems firmly built. I think some CSS styling you could make it pop a bit more. It's look is so basic that it reminds me of old-school webpages.
I also wanted to commend you for the responsive design. I had to finish my review on mobile and your site adapted well to my screen resolution which is excellent.
Poor
Site Structure:
It is not often I go for a converter but your structure works really nicely as far as I can see.
I had no trouble locating whatever converter had on my head. The menu works by dividing the items into a few categories. I am surprised miscellaneous seems to have well-formed subsections, why not add those sub-categories as part of the main ones? They seem a bit too buried the way they currently are. Miscellaneous I think, works better for things that you just can't put together in categories. For example, putting the clothes and shoe ones into one category to deal with wearables and fashion stuff.
To be honest I don't think you need second menus for Math, text and the like, since if we click on them the page loads with all the subsections. If you axed those sub menus I would also recommend you changing the term 'miscellaneous' to "other", "more..." or something that entices browsing the category. Miscellaneous sounds more like a disorganised drawer we throw files into but dread to open knowing the contents have no relation among themselves. However, you actually have sub-categories in there.
I like that you have a search system and an alphabetic index. Out of the two, the search function works the best for me. I insert foot and it tells me what conversions are available that uses this kind of measurement. It wouldn't hurt to make it more prominent. For example, it would be great having it on the home page, directly inviting me to type what I am looking for.
Overall, I find the site's structure to be very complete.
Excellent
The thing you liked best about the site: One website for any kind of conversion you will ever need.
The thing you liked least about the site: The redirecting pop-ups adverts.
Overall Mark:
Your website seems pretty useful. It is fast to understand and load the pages, It is just the aggressive advertising what really is bringing it down. There is a lot of effort into it and I really hope more people will give it a chance. I hope this review could be of help.
