What is the formula to calculate GST?

For GST exclusive (adding GST to base price): GST Amount equals Base Price multiplied by GST Rate divided by 100. Total Price equals Base Price plus GST Amount. For GST inclusive (extracting GST from total): Base Price equals Total Price multiplied by 100 divided by (100 plus GST Rate). GST Amount equals Total Price minus Base Price. Example: Base Rs 1,000 at 18% exclusive. GST equals Rs 1,000 times 18 divided by 100 equals Rs 180. Total Rs 1,180. Reverse: Rs 1,180 at 18% inclusive. Base equals Rs 1,180 times 100 divided by 118 equals Rs 1,000. GST equals Rs 180.

How do I calculate GST on MRP?

MRP (Maximum Retail Price) in India already includes GST. To find the GST component in an MRP, use the inclusive formula: Base Price equals MRP multiplied by 100 divided by (100 plus GST Rate). For example, if MRP is Rs 590 and GST rate is 18%, Base Price equals Rs 590 times 100 divided by 118 equals Rs 500. GST included in MRP is Rs 90. Never add GST on top of MRP — that results in double taxation. Use our free reverse GST calculator at mybillplease.com/tools/reverse-gst for instant MRP breakdowns.

How do I calculate CGST and SGST separately?

For intra-state supplies (buyer and seller in the same state), the total GST rate is split equally between CGST and SGST. If the GST rate is 18%, CGST is 9% and SGST is 9%. Calculate each separately: on Rs 10,000 taxable value, CGST equals Rs 10,000 times 9 divided by 100 equals Rs 900. SGST equals the same Rs 900. Total GST is Rs 1,800. For inter-state supplies, the full amount goes as IGST — there is no split. myBillPlease determines the correct split automatically from the GSTIN state codes.

What are the current GST rates in India 2026?

After the GST 2.0 reform effective September 22, 2025, India has three main GST slabs: 5% for essential goods like packaged food, medicines, and basic clothing; 18% as the standard rate for most goods and services including consumer electronics, furniture, and professional services; and 40% for luxury and sin goods like tobacco, aerated beverages, and luxury vehicles. The earlier 12% and 28% slabs were abolished. Several items moved to 0% including life insurance and health insurance premiums. Check our complete new GST rates list at mybillplease.com/blog/new-gst-rates-2026.

How do I calculate GST with a discount?

When a discount is given on the invoice, calculate GST on the price after discount, not before. Example: Product price Rs 10,000 with 10% discount. Discounted price equals Rs 9,000. GST at 18% equals Rs 9,000 times 18 divided by 100 equals Rs 1,620. Total invoice equals Rs 10,620. The invoice must show the original price, discount amount, taxable value after discount, and GST on the discounted value. If the discount is given after the invoice (post-sale), you issue a credit note instead of adjusting the original invoice.

Can I use a GST calculator for free?

Yes, mybillplease.com offers two free GST calculators that require no signup. The GST Calculator at mybillplease.com/tools/gst-calculator handles exclusive calculations — enter base amount and rate, get CGST, SGST, and IGST breakup instantly. The Reverse GST Calculator at mybillplease.com/tools/reverse-gst handles inclusive calculations — enter total amount including GST, get the base price and tax extracted. Both support all GST rates including the new 40% slab. Calculations are accurate to 2 decimal places as per GST rules.

How is GST calculated on services?

GST on services is calculated the same way as goods — the formula is identical. The difference is that services use SAC codes instead of HSN codes, and the place of supply for services is typically the location of the service recipient, not the delivery location. Most professional services like IT consulting, marketing, legal, and accounting are at 18% GST. Restaurant services are at 5% (non-AC in non-star hotels) or 18% (others). Calculate GST on the service fee using the exclusive formula and apply CGST plus SGST for intra-state or IGST for inter-state based on the client location.

GST Rates & Lists

How to Calculate GST: Formula, Examples, and Free Calculator 2026

Learn the GST calculation formula for both inclusive and exclusive prices. Step by step examples for 5%, 18%, and 40% rates with a free online calculator.

Amresh Kumar25 March 2026 · 6 min read · updated 6 September 2026

Close-up of a woman using a calculator and phone at her office desk.

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Key takeaways

Exclusive: GST = base price x rate / 100. Inclusive: base price = total x 100 / (100 + rate); GST is the difference.
Intra-state GST splits equally into CGST and SGST (9% + 9% at 18%); inter-state supplies charge the full rate as IGST.
After GST 2.0 the rates are 0%, 5%, 18% and 40%; the 12% and 28% slabs were abolished on September 22, 2025.
Calculate GST item by item at each item's rate, and always on the value after any invoice discount.

Two Types of GST Calculation

There are two ways to calculate GST — and confusing them is one of the most common billing mistakes in India. Exclusive calculation adds GST on top of the base price: 'My product costs Rs 1,000, how much GST do I add?' Inclusive calculation extracts GST from a total price: 'The MRP is Rs 1,180, how much of that is GST?' Both use simple formulas, and our free GST calculator handles both instantly.

After the GST 2.0 reform, India has three main rates: 5% (essential goods), 18% (standard rate), and 40% (luxury and sin goods). The old 12% and 28% slabs were abolished in September 2025. This tutorial covers the calculation formula for all rates with worked examples. Use the reverse GST calculator for inclusive calculations.

Step 1: GST Exclusive Calculation (Adding GST to Base Price)

This is the most common calculation — you know the price before tax and want to add GST.

Formula: GST Amount = Base Price × GST Rate / 100

Total Price = Base Price + GST Amount

Example 1 — 18% GST: You sell a laptop for Rs 50,000 (before tax). GST at 18% = Rs 50,000 × 18/100 = Rs 9,000. For intra-state: CGST = Rs 4,500, SGST = Rs 4,500. For inter-state: IGST = Rs 9,000. Total invoice = Rs 59,000.

Example 2 — 5% GST: You sell packaged food for Rs 200. GST at 5% = Rs 200 × 5/100 = Rs 10. CGST = Rs 5, SGST = Rs 5 (intra-state). Total = Rs 210.

Example 3 — 40% GST: A bottle of aerated drink for Rs 50. GST at 40% = Rs 50 × 40/100 = Rs 20. Total = Rs 70. This is the new luxury slab under GST 2.0.

Try it yourself: open the GST calculator, enter the amount, select the rate, and get instant results with CGST/SGST/IGST breakup.

Step 2: GST Inclusive Calculation (Extracting GST from Total Price)

When you see an MRP or a total amount and need to find out how much GST is included — this is the reverse calculation.

Formula: Base Price = Total Price × 100 / (100 + GST Rate)

GST Amount = Total Price - Base Price

Example 1 — 18% GST inclusive: A product has MRP Rs 1,180 including GST. Base Price = Rs 1,180 × 100 / 118 = Rs 1,000. GST = Rs 1,180 - Rs 1,000 = Rs 180. CGST = Rs 90, SGST = Rs 90.

Example 2 — 5% GST inclusive: Total amount Rs 525 including GST. Base = Rs 525 × 100 / 105 = Rs 500. GST = Rs 25. CGST = Rs 12.50, SGST = Rs 12.50.

Example 3 — 40% GST inclusive: Total Rs 700 including GST. Base = Rs 700 × 100 / 140 = Rs 500. GST = Rs 200.

Use our reverse GST calculator for instant inclusive calculations. It extracts the base price and GST amount from any total.

Quick Reference: GST Rates After GST 2.0

Effective from September 22, 2025

GST Rate	Common Items
0% (Exempt)	Fresh food, milk, curd, bread, life/health insurance (new under GST 2.0)
5%	Packaged food, soaps, medicines, footwear under Rs 1,500, economy air travel
18%	Most goods and services — electronics, furniture, IT services, telecom, restaurants
40%	Tobacco, aerated drinks, luxury cars above Rs 20L, sports bikes above 350cc
Abolished: 12%	Items moved to 5% or 18% — no longer applicable
Abolished: 28%	Items moved to 18% or 40% — no longer applicable

Step 3: Calculating GST on an Invoice with Multiple Items

Most invoices have multiple items at different GST rates. The calculation is item-by-item — you cannot average the rates.

Example invoice:

Item 1: T-shirt (HSN 6109) — Rs 800 × 2 qty = Rs 1,600 at 5% GST = Rs 80
Item 2: Laptop bag (HSN 4202) — Rs 2,000 × 1 qty = Rs 2,000 at 18% GST = Rs 360
Item 3: Wireless mouse (HSN 8471) — Rs 500 × 1 qty = Rs 500 at 18% GST = Rs 90

Invoice total: Subtotal Rs 4,100 + Total GST Rs 530 = Rs 4,630

Tax breakup (intra-state): CGST on 5% items = Rs 40. SGST on 5% items = Rs 40. CGST on 18% items = Rs 225. SGST on 18% items = Rs 225.

Each item's GST is calculated separately and then aggregated. The invoice shows the breakup by rate slab. This is how your GSTR-1 HSN summary table works — aggregating by HSN code and rate.

In myBillPlease, you add items to the invoice and the system calculates everything — per-item tax, per-rate totals, and the invoice grand total with correct CGST/SGST/IGST splits. Zero manual math.

Step 4: GST Calculation with Discounts

Discounts change the taxable value — and GST is calculated on the value AFTER discount, not before.

Rule: If the discount is mentioned on the invoice and linked to the supply, GST applies on the discounted value. If the discount is given later (post-sale), you issue a credit note.

Example: Product price Rs 10,000. Discount 10% = Rs 1,000. Taxable value = Rs 9,000. GST at 18% = Rs 1,620 (not Rs 1,800). Total = Rs 10,620.

Wrong way: Some businesses calculate GST on the full price (Rs 1,800) and then subtract the discount. This overcharges the customer and creates ITC discrepancies.

In myBillPlease, enter the discount per item or as a percentage — the system recalculates the taxable value and GST automatically. The invoice shows the original price, discount, and tax on the discounted amount — exactly as GST rules require.

Common GST Calculation Mistakes

Calculating GST on MRP

If MRP already includes GST (inclusive price), you must extract GST using the reverse formula. Adding GST on top of MRP results in double taxation. Use the reverse GST calculator.

Using Abolished 12% or 28%

After GST 2.0 (Sep 2025), the 12% and 28% rates no longer exist. Using them on invoices is non-compliant. Update your product catalog to the new 5%, 18%, or 40% rates.

Wrong Rounding

GST amounts should be rounded to 2 decimal places. For CGST and SGST, round each separately — don't round the total and split. Rs 127.50 becomes CGST Rs 63.75 + SGST Rs 63.75.

GST Before Discount

Always calculate GST on the price after applying the discount, not before. GST on pre-discount amount overcharges the customer and creates ITC mismatches.

Mixing IGST with CGST+SGST

An invoice must have either IGST or CGST+SGST — never both on the same invoice. If some items are inter-state and some intra-state, they need separate invoices.

Not Updating Post-GST 2.0

Hundreds of items changed rates in September 2025. If you haven't updated your product GST rates since then, every invoice has wrong tax. Review your catalog now.

Skip the Math — Use Our Free GST Calculator

Why calculate manually when you can get instant, accurate results?

GST Calculator: Enter any amount and GST rate. Get instant CGST, SGST, and IGST breakup for both exclusive and inclusive calculations. Free, no signup.

Reverse GST Calculator: Enter the total amount including GST. Instantly get the base price and tax extracted. Perfect for MRP-based pricing.

myBillPlease Billing: Save products with GST rates. Every invoice auto-calculates tax — per item, per rate slab, with correct CGST/SGST/IGST splits. Free forever plan with unlimited invoices.

Frequently asked questions

What is the formula to calculate GST?: For GST exclusive (adding GST to base price): GST Amount equals Base Price multiplied by GST Rate divided by 100. Total Price equals Base Price plus GST Amount. For GST inclusive (extracting GST from total): Base Price equals Total Price multiplied by 100 divided by (100 plus GST Rate). GST Amount equals Total Price minus Base Price. Example: Base Rs 1,000 at 18% exclusive. GST equals Rs 1,000 times 18 divided by 100 equals Rs 180. Total Rs 1,180. Reverse: Rs 1,180 at 18% inclusive. Base equals Rs 1,180 times 100 divided by 118 equals Rs 1,000. GST equals Rs 180.
How do I calculate GST on MRP?: MRP (Maximum Retail Price) in India already includes GST. To find the GST component in an MRP, use the inclusive formula: Base Price equals MRP multiplied by 100 divided by (100 plus GST Rate). For example, if MRP is Rs 590 and GST rate is 18%, Base Price equals Rs 590 times 100 divided by 118 equals Rs 500. GST included in MRP is Rs 90. Never add GST on top of MRP — that results in double taxation. Use our free reverse GST calculator at mybillplease.com/tools/reverse-gst for instant MRP breakdowns.
How do I calculate CGST and SGST separately?: For intra-state supplies (buyer and seller in the same state), the total GST rate is split equally between CGST and SGST. If the GST rate is 18%, CGST is 9% and SGST is 9%. Calculate each separately: on Rs 10,000 taxable value, CGST equals Rs 10,000 times 9 divided by 100 equals Rs 900. SGST equals the same Rs 900. Total GST is Rs 1,800. For inter-state supplies, the full amount goes as IGST — there is no split. myBillPlease determines the correct split automatically from the GSTIN state codes.
What are the current GST rates in India 2026?: After the GST 2.0 reform effective September 22, 2025, India has three main GST slabs: 5% for essential goods like packaged food, medicines, and basic clothing; 18% as the standard rate for most goods and services including consumer electronics, furniture, and professional services; and 40% for luxury and sin goods like tobacco, aerated beverages, and luxury vehicles. The earlier 12% and 28% slabs were abolished. Several items moved to 0% including life insurance and health insurance premiums. Check our complete new GST rates list at mybillplease.com/blog/new-gst-rates-2026.
How do I calculate GST with a discount?: When a discount is given on the invoice, calculate GST on the price after discount, not before. Example: Product price Rs 10,000 with 10% discount. Discounted price equals Rs 9,000. GST at 18% equals Rs 9,000 times 18 divided by 100 equals Rs 1,620. Total invoice equals Rs 10,620. The invoice must show the original price, discount amount, taxable value after discount, and GST on the discounted value. If the discount is given after the invoice (post-sale), you issue a credit note instead of adjusting the original invoice.
Can I use a GST calculator for free?: Yes, mybillplease.com offers two free GST calculators that require no signup. The GST Calculator at mybillplease.com/tools/gst-calculator handles exclusive calculations — enter base amount and rate, get CGST, SGST, and IGST breakup instantly. The Reverse GST Calculator at mybillplease.com/tools/reverse-gst handles inclusive calculations — enter total amount including GST, get the base price and tax extracted. Both support all GST rates including the new 40% slab. Calculations are accurate to 2 decimal places as per GST rules.
How is GST calculated on services?: GST on services is calculated the same way as goods — the formula is identical. The difference is that services use SAC codes instead of HSN codes, and the place of supply for services is typically the location of the service recipient, not the delivery location. Most professional services like IT consulting, marketing, legal, and accounting are at 18% GST. Restaurant services are at 5% (non-AC in non-star hotels) or 18% (others). Calculate GST on the service fee using the exclusive formula and apply CGST plus SGST for intra-state or IGST for inter-state based on the client location.