Estimating VRF Costs

For Chainlink VRF v2 to fulfill your requests, you must maintain a sufficient amount of LINK in your subscription balance. Gas cost calculation includes the following variables:

• Gas price: The current gas price, which fluctuates depending on network conditions.

• Callback gas: The amount of gas used for the callback request that returns your requested random values.

• Verification gas: The amount of gas used to verify randomness on-chain.

The gas price depends on current network conditions. The callback gas depends on your callback function, and the number of random values in your request. The cost of each request is final only after the transaction is complete, but you define the limits you are willing to spend for the request with the following variables:

• Gas lane: The maximum gas price you are willing to pay for a request in wei. Define this limit by specifying the appropriate keyHash in your request. The limits of each gas lane are important for handling gas price spikes when Chainlink VRF bumps the gas price to fulfill your request quickly.

• Callback gas limit: Specifies the maximum amount of gas you are willing to spend on the callback request. Define this limit by specifying the callbackGasLimit value in your request.

For Chainlink VRF v2 to fulfill your requests, you must have a sufficient amount of LINK in your consuming contract. Gas cost calculation includes the following variables:

• Gas price: The current gas price, which fluctuates depending on network conditions.

• Callback gas: The amount of gas used for the callback request that returns your requested random values. The callback gas depends on your callback function and the number of random values in your request. Set the callback gas limit to specify the maximum amount of gas you are willing to spend on the callback request.

• Verification gas: The amount of gas used to verify randomness on-chain.

• Wrapper overhead gas: The amount of gas used by the VRF Wrapper contract. See the Request and Receive Data section for details about the VRF v2 Wrapper contract design.

Because the consuming contract directly pays the LINK for the request, the cost is calculated during the request and not during the callback when the randomness is fulfilled. Test your callback function to learn how to correctly estimate the callback gas limit.

• If the gas limit is underestimated, the callback fails and the consuming contract is still charged for the work done to generate the requested random values.
• If the gas limit is overestimated, the callback function will be executed but your contract is not refunded for the excess gas amount that you paid.

Make sure that your consuming contracts are funded with enough LINK tokens to cover the transaction costs. If the consuming contract doesn't have enough LINK tokens, your request will revert.

You need to pre-fund your subscription enough to meet the minimum subscription balance in order to have a buffer against gas volatility.

After the request is complete, the final gas cost is recorded based on how much gas is used for the verification and callback. The actual cost of the request is deducted from your subscription balance.

The total gas cost in wei for your request uses the following formula:

(Gas price * (Verification gas + Callback gas)) = total gas cost

The total gas cost is converted to LINK using the ETH/LINK data feed. In the unlikely event that the data feed is unavailable, the VRF coordinator uses the fallbackWeiPerUnitLink value for the conversion instead. The fallbackWeiPerUnitLink value is defined in the coordinator contract for your selected network.

The final gas cost to fulfill randomness is estimated based on how much gas is expected for the verification and callback. The total gas cost in wei uses the following formula:

(Gas price * (Verification gas
              + Callback gas limit
              + Wrapper gas overhead)) = total gas cost

The total gas cost is converted to LINK using the ETH/LINK data feed. In the unlikely event that the data feed is unavailable, the VRF Wrapper uses the fallbackWeiPerUnitLink value for the conversion instead. The fallbackWeiPerUnitLink value is defined in the VRF v2 Wrapper contract for your selected network.

The maximum allowed callbackGasLimit value for your requests is defined in the Coordinator contract supported networks page. Because the VRF v2 Wrapper adds a gas overhead, your callbackGasLimit must not exceed maxGasLimit - wrapperGasOverhead.

The LINK premium is added to the total gas cost. The premium is defined in the coordinator contract with the fulfillmentFlatFeeLinkPPMTier1 parameter in millionths of LINK.

(total gas cost + LINK premium) = total request cost

The total request cost is charged to your subscription balance.

A LINK premium is then added to the total gas cost. The premium is divided in two parts:

• Wrapper premium: The premium percentage. You can find the percentage for your network in the Supported networks page.
• Coordinator premium: A flat fee. This premium is defined in the fulfillmentFlatFeeLinkPPMTier1 parameter in millionths of LINK. You can find the flat fee of the coordinator for your network in the Supported networks page.

(Coordinator premium
  + (total gas cost * (1 + Wrapper premium percentage)) = total request cost

This is an example calculation of a VRF request on the Ethereum network. The values for other supported networks are available on the Supported Networks page.

Estimate minimum subscription balance

You need to have the minimum subscription balance for your requests to be processed. This provides a buffer in case gas prices go higher when processing the request. The actual cost of the request is usually lower than the minimum subscription balance.

1. Calculate the total gas cost, using the maximum possible gas price for the selected gas lane, the estimated maximum verification gas, and the full callback gas limit:

   Gas cost calculation	Total gas cost
   Gas price x (Verification gas + Callback gas)
   500 gwei x (200000 + 100000)	150000000 gwei (0.15 ETH)

2. Convert the gas cost to LINK using the LINK/ETH feed. For this example, assume the feed returns a conversion value of Ξ0.004 ETH per 1 LINK.

   ETH to LINK cost conversion	Total gas cost (LINK)
   0.15 ETH / 0.004 ETH/LINK	37.5 LINK

3. Add the LINK premium to get the total maximum cost of a request:

   Adding LINK premium	Maximum request cost (LINK)
   Total gas cost (LINK) + LINK premium
   37.5 LINK + 0.25 LINK	37.75 LINK

This example request requires a minimum subscription balance of 37.75 LINK. Check the Max Cost in the Subscription Manager to view the minimum subscription balance for all your contracts. When your request is processed, the actual cost of the request is deducted from your subscription balance.

Estimate VRF request cost

This example reflects an estimate of how much a VRF request costs. Check Etherscan for current gas prices.

1. Calculate the total gas cost:

   Gas cost calculation	Total gas cost
   Gas price x (Verification gas + Callback gas)
   50 gwei x (115000 + 95000)	10500000 gwei (0.0105 ETH)

2. Convert the gas cost to LINK using the LINK/ETH feed. For this example, assume the feed returns a conversion value of Ξ0.004 ETH per 1 LINK.

   ETH to LINK cost conversion	Total gas cost (LINK)
   0.0105 ETH / 0.004 ETH/LINK	2.625 LINK

3. Add the LINK premium to get the total cost of a request:

   Adding LINK premium	Total request cost (LINK)
   Total gas cost (LINK) + LINK premium
   2.625 LINK + 0.25 LINK	2.875 LINK

This example request would cost 2.875 LINK, which is deducted from your subscription balance.

This is an example calculation of a VRF request on the Ethereum network. The values for other supported networks are available on the Supported Networks page.

Steps

1. Calculate the total gas cost:

   Gas cost calculation	Total gas cost
   Gas price x (Verification gas + Callback gas limit + Wrapper gas overhead)
   50 gwei x (90000 + 100000 + 40000)	11500000 gwei (0.0115 ETH)

2. Convert the gas cost to LINK using the LINK/ETH feed. For this example, assume the feed returns a conversion value of Ξ0.004 ETH per 1 LINK.

   ETH to LINK cost conversion	Total gas cost (LINK)
   0.0115 ETH / 0.004 ETH/LINK	2.875 LINK

3. Add the LINK premium to get the total cost of a request:

   Adding LINK premium	Total request cost (LINK)
   Coordinator premium + (Total gas cost x (1 + Wrapper premium percentage))
   0.25 LINK + (2.875 x (1 + 0))	3.125 LINK

This example request would cost 3.125 LINK.