Here is a tool I'm including in the upcoming update to the Measurement Conversions lesson app for Grade 6. It's a very simple, and abstract, measurement converter.
There are 3 basic parts to a conversion—or, at least, there is a way of deconstructing conversions into 3 parts—a conversion ratio and two values, which are equal to each other so long as we take units of measure into account. Wrangling those values, their collective structure and how they work together, and figuring out how to detect and work with known parts of the structure to make up for the unknown parts is really tough! Conversions are, in a real way, where ratio fluency and conceptual understanding come together first in Grade 6 (and beyond).
When you enter a positive value (integer or decimal) into either of the bottom boxes, along with a conversion ratio, (and then press Enter in one of those boxes), the converter will produce the other value. Entering two values in the bottom boxes and leaving the conversion ratio blank will make the converter calculate the conversion ratio. Where you press Enter—bottom left or bottom right—changes how the conversion ratio appears. But of course it's the same ratio. Why this is, is a question we ask students to think about in the activity.
And here are a couple of other questions.