To qualify for Carer's Allowance you must earn less than a weekly earnings limit (£128 from April 2020). However, any childcare costs or replacement care costs associated with working can be deducted from your earnings when calculating whether you qualify for Carer's Allowance.
You're allowed to deduct up to half your net earnings to reflect any payments you make to someone (other than a close relative) to look after your child or to look after the severely disabled person you normally care for while you're at work. This may reduce your earnings to a level where you qualify for Carer's Allowance.
For instance, you may earn £180 per week but have replacement care costs of £80 per week (money you pay to someone to look after the severely disabled person you normally care for while you're at work). In this case you would still qualify for Carer's Allowance, because your earnings after deductions would be below the weekly limit.
If you are using the calculator to look at your current situation the calculator allows for both childcare costs and replacement care costs. You will see a question about replacement care costs on the net earnings page.
For information on the other rules connected with Carer's Allowance please see qualifying for Carer's Allowance