GST Calculator Guide: Forward, Reverse, CGST + SGST + IGST Split (2025)

GST math is just two formulas — forward (add GST on top) and reverse (extract GST from a GST-inclusive price). Below: when to use which, the four slabs, and the CGST + SGST split that confuses every first-time invoice writer.

GST math comes up in three places: pricing decisions (what should the customer-facing MRP be?), invoice writing (how do I split CGST and SGST?), and reverse-engineering vendor quotes (the supplier said ₹1180 — what was the base price?). The actual math is high-school arithmetic. The tricky part is knowing whether you're going forward or reverse, and which split applies.

Forward GST: adding tax on top

You have a base price (what you'd charge before tax). You want the GST-inclusive total. Use this when:

Setting a price for a customer based on your cost + margin
Quoting a job where you'll add GST to the invoice
Computing the GST you'll owe on a B2B sale

GST   = Base × Rate%
Total = Base + GST

Example: base ₹1,000 at 18% GST → GST = ₹180, total = ₹1,180.

Reverse GST: extracting tax from an inclusive price

You see an MRP / inclusive price. You want to know how much of it is tax. Use this when:

Reading a vendor invoice that quotes a final price
Computing input tax credit on a purchase
Reconciling MRP-based pricing where GST is built in

Base = Total ÷ (1 + Rate/100)
GST  = Total − Base

Example: total ₹1,180 at 18% GST → base = 1,180 ÷ 1.18 = ₹1,000, GST = ₹180.

Use our free GST Calculator for both — it switches between forward and reverse with a tab.

The four standard slabs (and the zero rate)

Slab	Typical goods/services
0%	Essential food, milk, fresh vegetables, books, education services, healthcare
5%	Packaged food, footwear under ₹1000, life-saving drugs, basic services, rail/economy air travel
12%	Processed food, mobile phones, business-class flights, hotels ₹1000-₹7500/night
18%	Most goods and services — the default. Restaurants (non-AC), software, financial services, telecom
28%	Luxury (cars, motorcycles ≥350cc, AC restaurants), tobacco, aerated beverages — many also incur Compensation Cess

Find your product's rate via its HSN code using HSN Code Lookup, then refer to the official CBIC rate notification.

CGST + SGST split (intra-state)

For transactions where both parties are in the same state, the GST splits 50:50 between Central GST and State GST. So 18% becomes 9% CGST + 9% SGST, both shown as separate line items on the invoice.

The buyer claims input tax credit on each separately — CGST credit can offset CGST or IGST on outputs; SGST credit can offset SGST or IGST. They cannot cross-offset (CGST credit can't pay SGST owed, and vice versa).

IGST (inter-state)

For transactions where buyer and seller are in different states, the entire GST becomes Integrated GST (IGST). 18% IGST instead of 9% CGST + 9% SGST. The Centre collects, then shares with the destination state via settlement.

IGST credit is the most flexible — it can offset CGST, SGST, or further IGST on output. Companies preferring credit utilization sometimes intentionally route purchases through inter-state vendors.

UTGST (Union Territories)

For transactions in Union Territories without legislatures (Andaman, Lakshadweep, Daman & Diu, Dadra & Nagar Haveli, Chandigarh, Ladakh): replace SGST with UTGST. Same math, different revenue allocation. UTs with legislatures (Delhi, Puducherry, J&K) use SGST normally.

Worked examples by slab

5%: ₹500 packaged food → forward: GST ₹25, total ₹525. Reverse from ₹525: base ₹500, GST ₹25.
12%: ₹15,000 mobile phone → forward: GST ₹1,800, total ₹16,800.
18%: ₹50,000 software subscription → forward: GST ₹9,000, total ₹59,000.
28%: ₹8,00,000 car ex-showroom → forward: GST ₹2,24,000 + 1% Cess ₹8,000 = total ₹10,32,000.

Compensation Cess (the often-forgotten extra)

On top of the 28% slab, certain goods attract a Compensation Cess. Examples: cars (1-22% depending on engine size), tobacco (up to 290%), aerated beverages (12%), coal (₹400/ton). For these, your “effective rate” is GST + Cess.

For imports, IGST is calculated on (CIF + BCD + SWS), and Cess is calculated on the same assessable. See our Import Duty India Explained guide for the full landed-cost stack.

What people get wrong

Confusing “base” with “invoice value”. If a vendor quotes “₹1,180 inclusive of all taxes”, the base is ₹1,000 not ₹1,180. This matters for reverse-charge mechanism.
Splitting 18% as 9% + 9% for inter-state transactions. Inter-state is full 18% IGST, not split. Mistake in invoicing causes credit-utilisation issues for the buyer.
Missing Compensation Cess. 28% on a luxury car is ~28% — but the actual all-in tax often hits 36-50% with Cess. Include in landed-cost calculations.
Forward-then-reverse rounding. ₹1,000 × 18% = ₹180 (forward), ₹1,180 ÷ 1.18 = ₹999.99 (reverse). Sub-rupee differences from float math; round consistently.

Plug your numbers into the GST Calculator — it handles forward, reverse, slab selection, and CGST/SGST split automatically.