What's the difference? – Feasible, doable, viable and practical

The words feasible, doable, viable and practical all have similar meanings. While they are often used interchangeably by native speakers, and have a lot of overlap, there are subtle differences between them that can help you choose the most appropriate one for the situation.

Feasible and doable

Feasible and doable have similar meanings. If something is feasible or doable it means it can be done; it's possible to do it – at least in theory.

The main difference between these words is that doable feels more informal. We are more likely to use doable when talking about hands-on work, such as the execution of tasks or more mundane (everyday) issues such as arranging times.

• Is Monday 3 p.m. doable for you?
• Sure, 10 grams is doable by Friday.
• That's doable, but is would require us to add an extra FTE to the team for the remainder of the project.
  

Feasible, on the other hand, is more formal. It's more likely to be used when referring to higher-level things like proposals, concepts, approaches, solutions.

• We've gone through the plan carefully and it looks feasible to us.
• Given the time constraints, we don't think it's feasible to develop a novel route for this target.
• It may not be feasible to change the plan at this late stage.
  

The opposite is unfeasible or infeasible (both are acceptable), though it's also common to use not feasible.

If something is impossible, it's common to use unfeasible or not feasible to express that in a more diplomatic way. Undoable is not so commonly used, but you can say something is not doable.

• I'm afraid it's unfeasible to produce double the amount this week. We will have to order more starting material.
• I have a meeting from 2-4 p.m., so I'm afraid 3 p.m. is not doable for me.
  

Practical

If something is practical, it means it is likely to be effective or successful in reality (i.e., not just in theory, but in practice). Practicality takes into account real-world considerations such as resources, time, and logistics.

Many things are feasible but not practical. For example, it's feasible to build a house made of gold, but that wouldn't be very practical – it would be prohibitively expensive and difficult to insulate. It's feasible to lose weight by eating nothing but potatoes, but that's not a practical solution for most people.

Here are some examples where practical is the best choice of word:

• The photo-redox route isn't practical for such a large-scale reaction.
• It's feasible to increase the yield to 98%, but that might take a couple of weeks. Given the time constraint, it would be more practical to work with the lower yield and increase the batch size.
• Both options are feasible, but option B is more practical, since it doesn't involve the handling of any toxic materials.
• For practical reasons, we changed the order and started with the targets in list B.
  

The opposite is impractical, though it's also common to use phrases like "may not be practical" to convey this meaning in a more diplomatic way:

• It might not be practical to scale up to kilogram scale using this method.
  

Viable

If something is viable, it is able to succeed or work as intended. For example, a viable plan is a plan that is capable of succeeding (just like a feasible plan). We are more likely to use viable than feasible in these situations:

Viable is often used when discussing possible options and alternatives:

• We have three viable options.
• The only viable alternative is to produce the material in-house.
• We found two viable routes in the literature.
  

Viable is also used in an economic or business sense:

• It's (not) economically/commercially/financially viable.
• A viable business / a viable business model
• We have a viable plan to increase revenue by 20% in the next quarter.
  

Here are some examples of sentences where viable is the best choice:

• We need to increase the yield for this to be a viable route for scale-up.
• We could make the intermediate in-house, but it would be more economically viable to source it from a local supplier.
• I'm afraid we were unable to find any viable alternatives, so we've decided to focus all our energy on this approach.
  

Note that there is a lot of overlap among these words, and often more than one choice is possible for a given sentence. But hopefully these guidelines can help you decide the most appropriate word for your situation.

To see some common collocations of these four words, check out this post:
Viable, Practical, Feasible, Doable