Question: If your salary is $1000 and you take home $800, what is your tithe?
First, let me say that I am glad that you are even asking this question because it shows that you have a heart and willingness to give. As Christians, we are called to support the ministry of the local church and to be a blessing to others financially. Every Christian should be regularly contributing to their church. Through financial giving you are supporting the work, ministry, and mission of the church both locally and globally. The word we generally use for this is tithe which means “a tenth.” The tithe is seen in OT before the law was given when Abraham gave a tithe to the priest Melchizedek and then was part of the giving practices of the people of Israel. Malachi 3 gives us one of the clearest pictures of the importance of bringing a tithe and here we are told by God to test him and see if he doesn’t bless our giving. In the NT Jesus affirms the tithe to the Pharisees in Matthew 23, by saying “These things you should have done.” The New Testament practice of giving is to be both “generously and cheerfully” and I think that the Apostles probably had the tithe in mind when they instructed in this way.
To get to your question, I am not sure the Bible says explicitly. Under the old covenant system I think they would have given their tithe pre-tax and with the other offerings they brought would’ve probably given closer to 20% of their income. When I think of giving, I like to ask myself a couple of questions. Am I seeking to give generously or the least I can get away with? This tells me a lot about my heart. I think a healthy believer should give consistently, generously, joyfully, and at times sacrificially. If I am struggling to give, why? Have I made poor choices that are prohibiting my ability to give? If so I need to remedy those. Does my heart love the pleasures of this world too much? If so, I need to repent and seek first the kingdom of God. Do I put too much trust in the money for security? If so, I need to realize that there is no security in the things of this world, but that my only ultimate security is in Christ and I need to trust in Him. I probably haven’t answered your question as precisely as you would have liked, but believe that the idea of the tithe should be the principle we use to start our practice of giving, and if we seek to give generously, cheerfully, and sacrificially that the Holy Spirit will guide our giving.
For His glory and your joy,