Checking API Usage

1) Below are the ways by which you can access the report to check API Usage for the organisation-

a) System Overview

Go to Monitor | System Overview and there you will find the API REQUESTS, LAST 24 HOURS. This shows you how many API calls you've made in the last 24 hours including today. For example, if you are viewing this on Monday at 2:30 PM, it'll show you the calls made since Sunday at 2:30 PM

b) Through Salesforce Report

Click the Reports tab and view the "API Usage Last 7 Days" report under the Administrative Reports folder. This will show you how many calls each app/person made in the last 7 days.

shortcut step->

put this URL in for your instance.  It's how I found the API report.https://na1.salesforce.com//00O?rt=104&retURL=%2F00O&c=UN&c=FULL_NAME&c=EM&c=CID&c=TS&c=CC&duel0=FULL_NAME%2CUN%2CEM&scope=organization&details=yes

Replace the na1 with your instance. 

c) You can check that in setup: 

In quick find type "Company Information". There you have an overview of your daily limits, including Streaming API:

2) Now let us see what the LIMITS for API Request in various types of org are

Concurrent API Request Limits here are the allocations for various types of orgs for concurrent requests (calls) with a duration of 20 seconds or longer:

Org Type	Limit
Developer Edition and Trial orgs	5
Production orgs and Sandbox	25

The following table lists the allocations for the total API requests (calls) per 24-hour period for an org.

Salesforce Edition	API Calls Per License Type	Total Calls Per 24-Hour Period
Developer Edition	N/A	15,000
Enterprise Edition Professional Edition with API access enabled	Salesforce: 1,000 Salesforce Platform: 1,000 Force.com - One App: 200 Customer Community: 0 Customer Community Login: 0 Customer Community Plus: 200 Customer Community Plus Login: 10 Partner Community: 200 Partner Community Login: 10 Lightning Platform Starter: 200 per member for Enterprise Edition orgs Lightning Platform Plus: 1000 per member for Enterprise Edition orgs	15,000 + (number of licenses X calls per license type), up to a maximum of 1,000,000
Unlimited Edition Performance Edition	Salesforce: 5,000 Salesforce Platform: 5,000 Force.com - One App: 200 Customer Community: 0 Customer Community Login: 0 Customer Community Plus: 200 Customer Community Plus Login: 10 Partner Community: 200 Partner Community Login: 10 Lightning Platform Starter: 200 per member for Unlimited and Performance Edition orgs Lightning Platform Plus: 5000 per member for Unlimited and Performance Edition orgs	15,000 + (number of licenses X calls per license type)
Sandbox	N/A	5,000,000

3) What counts towards my API limit?

SOAP and REST API calls (which include Bulk API calls) are counted against an organization API call limit.

3. How can I reduce my usage of API calls?

There are different approaches to reduce the amount of API calls consumed by a given API client, which include (but are not limited to) the following:
a) caching on the client side,
b) sending cumulative changes, rather than incremental changes,
c) resort to the Bulk API to mass upload/update records, instead of using the REST API to upload/update the same records.