I can not figure how to add the drive size into this, Given the desired size of a new file, calculate how much the available disk size will be reduced - operating-system

Question

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtMath>
#include <QMessageBox>
MainWindow::MainWindow(QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
{
ui->setupUi(this);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_pushButton_clicked()
{
}
//File size code
void MainWindow::on_btnMaxFileSize_clicked()
{
int cs = ui->txtClusterSize->toPlainText().toInt();
int si=ui->txtSingleIndirect->toPlainText().toInt();
int di=ui->txtDoubleIndirect->toPlainText().toInt();
int ti=ui->txttripleIndirect->toPlainText().toInt();
int ps=ui->txtPointerSize->toPlainText().toInt();
qlonglong max_file_size=(di *cs) + (si * (cs/ps)) +(di* qPow((cs/ps),2) * cs) + (ti*qPow((cs/ps),3) * cs);
QMessageBox dial;
dial.setText("Max File Size: " + QString::number(max_file_size) + " bytes");
dial.exec();
}
\ used this code to make a GUI APPLICATION TO MAKE the user to enter the characteristics of an i-node and asecondary memory device, i.e. total drive size, cluster size, number of direct links, etc