For 2024, a Solo 401(k) allows both spouses to contribute if they are both earning income from the business. Each spouse can make employee deferrals of up to $23,000 (or $30,500 if they are over 50). In addition, as the employer, the business can contribute up to 25% of compensation for each spouse, with a combined contribution limit (employee plus employer) of $73,500 per person or $146,000 total for the couple, subject to income levels and IRS guidelines.