What percentage of my income may I donate and still count on my income tax?

[DoninGA]

There is no limit to the amount of cash you can donate to an eligible charitable organization.  The donation is reported as an itemized deduction on Schedule A of your federal tax return.  However, if the total amount of itemized deduction is less than the Standard Deduction for your filing status the itemized deductions will have no affect on your tax return.

Standard deductions for 2021

• Single - $12,550 add $1,700 if age 65 or older
• Married Filing Separately - $12,550 add $1,350 if age 65 or older
• Married Filing Jointly - $25,100 add $1,350 for each spouse age 65 or older
• Head of Household - $18,800 add $1,700 if age 65 or older

[Mike9241]

however, if your itemized dductions exceed your taxable income, then you get no current tax benefit but there could be a charitable contribution carryover.

the amount of donations deductible is subject to various AGI limitations

1) 60% of AGI for cash donations to an organization qualifying as a 50% limit organizations

2) 50% of AGI for donations of property other than capital gain property to 50% limit organizations

3) 30% of AGI for donations of capital gain property to 50% limit organization or donations other than capital gain property to non 50% organizations 

4) 20% of AGI for donations of capital gain property to non-50% organiztions

IRS website to search for charities and there % limits also IRS pub 78

https://www.irs.gov/charities-non-profits/search-for-tax-exempt-organizations#pub78 

[Opus 17]

Charities are classified as "20% limit", "30% limit" and "50% limit" organizations.  That means, roughly speaking, you can donate 50% of your income to a 50% charity and take a deduction, but if you donate more, the unused portion of the deduction is carried forward to future tax years.  If you donate in the same year to 20%, 30% and 50% charities, the limits get applied in a confusing way.  You can read more about it here.

https://www.irs.gov/pub/irs-pdf/p526.pdf

Even though the top category is called "50% charities", the actual limit for cash contributions was temporarily raised to 60% through 2025.