Welcome to conversion.org, a user friendly website, where you can find conversion calculator for (currently) 3 categories of measurement units. Conversion.org provides several ways to enter source unit and high precision calculation. You can generate your own conversion charts, ready to copy into spreadsheet application. Newest category: area (2017-04-22), next category, under construction: volume.
Where it is possible (the definition between two units is exactly definied), there is option for converting with high precision!
Read more on precision page.