How GST is calculated

Below is a step‑by‑step learning guide to understand how GST is calculated in India, written in a simple → practical → invoice-level way. This is ideal for beginners, audit/compliance work, or invoice verification.

---

✅ GST in India — Step‑by‑Step Learning

STEP 1: What is GST?

GST (Goods and Services Tax) is a destination‑based indirect tax levied on the supply of goods and services in India.

It replaces multiple old taxes like VAT, Service Tax, Excise, etc.

---

STEP 2: Types of GST (MOST IMPORTANT CONCEPT)

GST calculation depends on where the seller and buyer are located.

✅ Case 1: Intra‑State Supply (Same State)

If seller and buyer are in the same state:

👉 GST is split into:

• CGST (Central GST)
• SGST (State GST)

Example:
Seller: Karnataka
Buyer: Karnataka

---

✅ Case 2: Inter‑State Supply (Different States)

If seller and buyer are in different states:

👉 Only IGST is charged

Example:
Seller: Karnataka
Buyer: Maharashtra

---

STEP 3: GST Rate Structure

Common GST slabs in India:

GST Rate	Used for
0%	Essential goods
5%	Basic items
12%	Standard goods
18%	Most services & IT items
28%	Luxury items

✅ Most services & B2B invoices use 18% GST

---

STEP 4: Identify the Taxable Value

GST is calculated on the taxable value, which includes:

✅ Product / Service price
✅ Packing charges
✅ Freight (if charged)
✅ Any additional fees

❌ GST itself is NOT included in taxable value

---

STEP 5: GST Calculation – Core Formula

✅ Basic Formula

GST Amount = Taxable Value × GST Rate

Invoice Total = Taxable Value + GST

---

STEP 6: Practical Calculation Examples

---

✅ Example 1: Intra‑State (CGST + SGST)

Taxable Value: ₹10,000
GST Rate: 18%

Split:

• CGST = 9%
• SGST = 9%

Calculation:

CGST = 10,000 × 9% = ₹900

SGST = 10,000 × 9% = ₹900

✅ Total GST = ₹1,800
✅ Invoice Total = ₹11,800

---

✅ Example 2: Inter‑State (IGST)

Taxable Value: ₹10,000
GST Rate: 18%

Calculation:

IGST = 10,000 × 18% = ₹1,800

✅ Invoice Total = ₹11,800

---

STEP 7: How GST Appears on an Invoice

✅ Intra‑State Invoice

Taxable Value : ₹10,000

CGST @9%      : ₹900

SGST @9%      : ₹900

--------------------

Invoice Total : ₹11,800

✅ Inter‑State Invoice

Taxable Value : ₹10,000

IGST @18%     : ₹1,800

--------------------

Invoice Total : ₹11,800

---

STEP 8: Input Tax Credit (ITC) – Business Use

If you are GST registered, GST paid can be claimed back as credit.

Example:

• You pay ₹1,800 GST on purchase
• You collect ₹2,700 GST on sale

✅ Net GST payable = ₹900

Output GST – Input GST = GST payable

---

STEP 9: Reverse Charge Mechanism (RCM) – Advanced

In some cases, buyer pays GST instead of seller, for example:

• Unregistered vendor
• Certain legal / transport services

✅ Buyer pays GST directly to government
✅ Buyer can still claim ITC (if eligible)

---

STEP 10: Common GST Mistakes (Audit Red Flags 🚨)

❌ Wrong GST type (IGST instead of CGST+SGST)
❌ Wrong GST rate
❌ GST calculated on total including GST
❌ Seller not GST registered but charging GST
❌ Missing GSTIN on invoice

---

✅ One‑Page GST Calculation Checklist (Audit‑Ready)

✔ Seller GSTIN present
✔ Buyer GSTIN present
✔ Correct state comparison
✔ Correct GST rate
✔ Correct split (IGST vs CGST/SGST)
✔ Math accuracy
✔ Invoice total matches calculation