Bring consistency in the units drop-down. Either all lowercase or all uppcase or First capital and rest small. Decide what's best for the consumer of this product.
what was your logic of placement of different options on the units? Was there any specific order you followed (Gram, kilogram, milligram, tons)? Think from a customer perspective.
mobile responsive needs to be worked on as per previous comments. The grey box of unit converter should adjust to the screen size.
Form validation -
1) negative numeric inputs shouldn't give result.
2) non numeric should be invalid.
what do you think of formatting the result, as some inputs are giving 20 decimal places output while some are 3-4. Decide on a logic for this.
is it a one input form or two input form? I am able to edit result too, shouldn't be there right? Incase, id you want to give an option for formatting the result, what will you do?