OPINION
Appellant Deborah Chenault, f/k/a Deborah Paul, appeals the trial court’s order affirming child support arrearages in her favor against appellee Patricia Banks, independent executrix for the estate of Horace A. Paul, Jr., of $5,105, rather than the over $90,000 in arrearages and interest she requested. In five issues, she claims the trial court abused its discretion in calculating arrearages and in failing to award her interest. We agree, and we reverse and remand.
BACKGROUND
Chenault and Paul, who is now deceased, were divorced on June 3, 1985. Paul was ordered to pay Chenault $300 per month in child support until their son turned eighteen, graduated from high school, or was otherwise emancipated. The decree was later modified to specify that child support payments were to be made through the child support registry. This is the only modification the trial court ever made to the child support obligation.
On November 6, 2006, Chenault filed a petition under Family Code section 157.263 to enforce child support and confirm arrearages against Patricia Banks, who is Paul’s sister and executrix of his estate. Chenault alleged that Paul had paid only $21,900 of his $57,300 child support obligation, and therefore she sought to recover the balance, plus interest, against his estate. Banks disputed Che- *189 nault’s calculation and brought forth evidence of additional payments to Chenault of child support and other items for their son, as well as at least one loan to Che-nault. Banks also submitted evidence that Paul had paid $18,176.31 directly to Marine Military Academy (“MMA”), a private boarding school that then* son had attended for one year.
After a bench trial, the trial court issued a letter to the parties stating that it found Paul’s total child support obligation was $46,800 and that Paul had paid child support in the amount of $41,695.36, which included $18,175.36 paid to MMA, 1 for a total arrearages amount of $5,105. Other than stating that the $41,695.36 included money paid to MMA, the trial court did not explain how it derived any of its calculations. The trial court also did not specify an amount of interest, stating “any interest is calculable only by computer and counsel are requested to make that calculation, or, agree to an amount of interest.” Less than a month later, the trial court issued a judgment for child support ar-rearages that awarded Chenault $5,105 in child support arrearages with no interest. The judgment did not repeat the other numbers that were included in the earlier letter.
ANALYSIS
A trial court’s decision to grant or deny the relief requested in a motion for enforcement is reviewed for an abuse of discretion.
See Beck v. Walker,
In calculating child support ar-rearages, the trial court’s discretion is very limited.
See Attorney Gen. v. Stevens,
A. Arrearages
In issues one, three, four, and five, Che-nault complains that the trial court improperly calculated the amount of child support arrearages. She claims the trial court erred in giving child support credit for the payments to MMA and in calculating the remaining arrearages.
1. Marine Military Academy
The evidence at trial showed that Chenault agreed with Paul that their son could attend MMA for one year and that Paul agreed to pay the entire cost. Banks testified that Chenault and Paul further agreed that Paul paying the cost of MMA would be considered child support, but Chenault testified that she did not so agree and that she and Paul had never discussed the issue.
Banks contends that because the trial court did not make findings of fact, we cannot determine that the trial court actually gave child support credit for the MMA payments. Banks insists that we cannot consider the trial court’s calculation in its prejudgment letter because it is not an official fact finding and therefore not binding.
See Cherokee Water Co. v. Gregg County Appraisal Dist.,
We conclude that to the extent it gave a child support credit for the amount paid to MMA, the trial court abused its discretion. The trial court plays an integral role in child support proceedings to ensure the protection of the child’s best interests.
See Williams v. Patton,
Banks argues that, independent of the enforceability of the alleged agreement, child support credit for the MMA payments is supportable based on the affirmative defense of voluntary relinquishment. The Family Code provides that “[a]n obligor may plead as an affirmative defense in whole or in part to a motion for enforcement of child support that the obli-gee voluntarily relinquished to the obligor actual possession and control of a child.” Tex. Fam.Code Ann. § 157.008(a) (Vernon 2008). Because voluntary relinquishment is an affirmative defense, the obligor has the burden of proof.
Beck,
Banks argues that because Paul paid for MMA, the trial court could have found that Chenault voluntarily relinquished actual possession and control of her son to Paul by allowing him to attend MMA. We reject this argument. The obligor must prove that the obligee affirmatively agreed to relinquish possession and control of the child to the obligor.
See
Tex. Fam.Code Ann. § 157.008(a);
Leighton v. Court,
For these reasons, we conclude the trial court abused its discretion to the extent it credited Paul’s payments to MMA toward his child support obligations.
2. Other Arrearages Calculations
Based on the uncontested evidence and the terms of the divorce decree, Paul was obligated to pay child support from July 1, 1985 to May 1, 2001, which is 191 months, at $300 per month, for a total child support obligation of $57,300. If the trial court gave a credit of $18,176.31 for the MMA payments, then considering its total arrearage judgment of $5,105, we im *192 ply that the trial court gave Paul another $34,018.69 in credit toward his child support obligation. If the trial court did not credit the MMA payments as child support, we imply the trial court gave Paul $52,195 in child support credits.
Chenault argues that the trial court abused its discretion because the evidence is insufficient to support the trial court’s ultimate conclusion that Paul owed only $5,105 in child support arrearages. Banks argues that because the evidence regarding child support payments was disputed and the amount the trial court found was within the range of the figures argued by the parties, the trial court acted within its discretion. We disagree with Banks’s analysis.
Banks presented evidence of every payment from Paul to Chenault she could document, including not only child support but personal loans and money for other items such as Christmas gifts, an amusement park season pass, pictures, insurance, a car, and a class ring. We do not know exactly how the trial court calculated the child support credits it gave. If the trial court gave credit for these non-child support items, that would constitute an abuse of discretion because these items are not provided in the Family Code as a basis for decreasing child support arrearages.
See
Tex. Fam.Code Ann. § 157.262(a);
Medrano v. Medrano,
Having concluded that the trial court abused its discretion in its calculation of child support arrearages, we sustain Che-nault’s first, third, fourth, and fifth issues.
B. Interest
In her second issue, Chenault complains that the trial court abused its discretion in failing to award interest on her child support arrearages judgment. The Family Code provides that if a motion for enforcement of child support requests a money judgment for arrearages, the court “shall confirm the amount of arrear-ages and render one cumulative money judgment” that includes “interest on the arrearages.” Tex. Fam.Code Ann. § 157.263(a), (b)(3) (Vernon 2008). The interest rate on each unpaid month of child support is either six or twelve percent, depending on the date of the unpaid payments and the date the unpaid support payments are judicially confirmed.
See id.
*193
§ 157.265;
In re M.C.C.,
The trial judge explained during the hearing on the motion for entry of judgment that “I think [interest is] uncertain .... and incapable of calculation due to the facts of the case.” However, interest on child support arrearages is inherently calculable because an arrearages determination must be based on specific unfulfilled child support obligations. Determining the date and amount of the unpaid obligation is necessary to calculate both the arrearage and the interest. . In its prejudgment letter, the trial court requested that the parties calculate the interest total but did not give them the necessary information to do so. Though the trial court was not required to provide findings of fact because Chenault’s request was not timely, her failure to do so did not forfeit her right to interest. The trial court had no discretion not to award interest, even without a timely request for findings of fact.
See Herzfeld,
CONCLUSION
The trial court abused its discretion in making its child support arrearages calculation and in failing to perform its mandatory duty of awarding interest on confirmed child support arrearages. We reverse the trial court’s judgment and remand for further proceedings consistent with this opinion.
Notes
. These findings are at odds with the undisputed evidence showing a total child support obligation of $57,300 (not $46,800) and payments to MMA of $18,176.31 (not $18,175.36).
. Chenault filed a request that the trial court make findings of fact and conclusions of law, but she failed to file a timely notice of overdue findings of facts and conclusions of law, and thus the trial court was not required to make them. See Tex.R. Civ. P. 296-299.
. These figures are based on the evidence presented, rather than the calculations in the trial court's prejudgment letter to the parties. Considering the figures in the letter does not change our analysis because the letter contains an undisputably incorrect total child support obligation, the amount credited for payments to MMA does not match the undisputed evidence of payments made, and the amount of other child support credited also cannot be derived from any combination of the figures and testimony presented.
