The 50/30/20 split treats your take-home (post-tax, post-PF) income as the budget base. 50% for needs (rent, EMIs, utilities, groceries, transport, insurance, school fees, minimum debt payments - all the non-negotiables). 30% for wants (dining out, OTT subscriptions, gadgets, weekend trips, hobbies, gifts). 20% for savings (mutual fund SIPs, additional retirement contributions, emergency fund top-ups, accelerated debt repayment beyond minimums).
The 'needs' bucket should include only true non-negotiables. A specific gym membership might feel like a need but is technically a want. The home-loan EMI is a need; an extra prepayment is a want or a save. Honest categorisation is essential because under-classifying needs (treating wants as needs) is the most common reason budgets fail.
Saving and investing are different but related. The 20% savings bucket combines emergency fund accumulation, debt repayment beyond minimums, and long-term investment SIPs. The right mix within savings depends on where you are: building an emergency fund first if you don't have 3-6 months of expenses set aside, accelerating high-rate debt next if you have it, and routing everything into long-term SIPs once those two foundations are in place.